Follow a fearless team of scientists as they venture into blue holes—underwater caves that formed during the last ice age, when sea level was nearly 400 feet below what it is today. These caves, little-known treasures of the Bahamas, are one of Earth's least explored and most dangerous frontiers. The interdisciplinary team of biologists, climatologists, and anthropologists discover intriguing evidence of the earliest human inhabitants of the islands, find animals seen nowhere else on Earth, and recover a remarkable record of the planet's climate.

Nancy Albury, Kenny Broad, Jill Heinerth, Brian Kakuk, Michael P. Pateman, Wes C. Skiles, David Steadman, Peter K. Swart

These red pointers, are disclosed below...

Extreme Cave Diving 52:57 NOVA Season 4

NOVA | Trailer

628 MB MP4 iTunes

PBS Airdate: February 9, 2010

NARRATOR: In the Bahamas islands, the world's best cave divers plunge into the abyss. They're exploring underwater caves, littered with mysteries and divers who didn't get out alive.

WES C. SKILES (Director of Photography, Karst Productions):Per attempt, this is the most dangerous sport on Earth.

NARRATOR: They face cave-ins, blackouts, panic. They do it for the rush, they do it to be first, and they do it to find ancient bones.

These caves are blue holes, liquid time capsules where the past stares right back at you.

Now, extreme divers try to solve the mystery of a lost world. But can they survive this alien place, under pressure, underwater, underground? Extreme Cave Diving , right now on this Nova/ National Geographic special.

In the Caribbean Sea, 60 miles off the coast of Florida, there is a paradise known as the Bahamas. This is a place full of attractions; they draw millions of tourists each year. But these visitors aren't here for sun or sand, though they may be in for a bit of a gamble. They've come to search for a lost world. They're extreme explorers, astronauts of an inner space called blue holes.

Blue holes get their name from the dark blue of their depths. And, while they don't look like much at the surface, what seems like a small pond can go thousands of feet down and spread out into a maze of underwater passages and tunnels. The mystery of their depths beckons the able and the foolhardy alike. On average, 20 divers die each year in caves like this.

But to these explorers, it's worth the risk, because blue holes aren't just spectacular, some preserve the past like a liquid time capsule. Diving them will take the explorers back thousands of years, where there are hints of an ancient, lost Bahamas.

What did this world look like then and who lived here? Now, an unprecedented expedition pairs expert cave divers with world-class scientists to find out.

KENNY BROAD (University of Miami) : There's two options of getting in here right now; one is jump, and I think the second one is jump.

NARRATOR: National Geographic explorer, Kenny Broad, an anthropologist, is the expedition leader.

KENNY BROAD: We are trying to get a window into what the past was like, the living past, the geologic past. And you can jump into a blue hole and make finds that change our picture of these islands.

NARRATOR: They'll do that by exploring seven blue holes, in just three weeks, on the islands of Andros and Abaco. The team has come here, because the Bahamas may have more blue holes than anywhere else in the world.

The potential for discovery is high, the risks, even higher. So Kenny has turned to Brian Kakuk to lead the dives. Brian is one of the world's most experienced cave divers.

BRIAN KAKUK (Science Diver, Bahamas Caves Research Foundation):We just want to see what's around that corner. We don't want to stop. Cave divers live to change the map, and, occasionally, that can get us into trouble.

NARRATOR: Renowned photographer, Wes Skiles, heads up a film crew who'll deliver images to document the scientific finds.

WES SKILES: I'm the eyes of the scientists and the explorers. I know that what I'm shooting is bringing this back to be shared in very important ways, and that's thrilling.

NARRATOR: Their first task is a shakedown dive. They'll test out the gear and get all the divers prepared for the treacherous labyrinths below.

The first thing dive leader Brian Kakuk does when he gets into the hole is lay down a guide line. In cave diving, laying line is the first rule of survival.

KENNY BROAD: Without that guideline, your odds of coming out are very slim, and that's what kills most people.

NARRATOR: Though they can appear crystal clear, blue holes are full of silt. When disturbed, it rises from the floor, or even falls from the ceiling, and blacks everything out.

KENNY BROAD: In this place, it's extremely dense, black, organic matter, so it's difficult to work in. Just the pressure wave of your body pushing the water in front of you tends to stir stuff up. You have to count on coming out in zero visibility, with your finger on that line, feeling your way out.

NARRATOR: But even that lifeline can turn against you.

WES SKILES: ...line, an inanimate object, a piece of nylon string: you put it underwater, in a cave, and it comes alive, and it's devious and evil. It's out to kill you.

BRIAN KAKUK: We have lots of stuff on us, and all of those things can become entrapped in the line or caught in the line, if we get too close to it or are not paying attention to it.

NARRATOR: If they do get tangled up, cave divers carry knives, usually more than one. They like backups, and they like their backups to have backups.

KENNY BROAD: Alright, just to give you just a little sense of the redundancy: so you've got knife, knife, another knife.

It's actually a parachute cutter, so...Shhhh, my mom's favorite kitchen knife. Don't tell her.

NARRATOR: Backup is second nature in this type of diving, where caves that look solid can suddenly fracture at a bubble's weightless touch. Only minutes into the dive, it happens. Brian turns around to see chunks of ceiling fall on Kenny. Kenny loses the line.

BRIAN KAKUK: It was just raining down crumbly rock, and he almost disappeared.

WES SKILES: We're talking about avalanches.

KENNY BROAD: I couldn't read any of my gauges.

NARRATOR: While filming, Wes is pummeled by the debris.

WES SKILES: Once it went to completely zero vis, I was trapped in there.

KENNY BROAD: Next time you're in your house, turn off all the lights, put a bag over your head and try to find your way out. Your heart rate will go up, and it takes awhile.

NARRATOR: Brian is still on the line. Kenny grabs his leg, and they escape, covered in debris.

KENNY BROAD: After the limestone rain shower, we decided just to get all the sediment off of us, so we did a couple barrel rolls, just, kind of, let things hang loose.

NARRATOR: It's just a small taste of what can go wrong.

WES SKILES: Well, per attempt, this is the most dangerous sport on Earth.

I see dead people that are diving with me. I look at people and go, "That person is a potential fatality."

NARRATOR: And down here, with the slightest mistake, potential becomes reality. Out of nowhere, a human arm bone appears; next, a flashlight; then a knife, strapped to the leg of a '70s-era wetsuit; an orange sleeve and a headless torso, the remains of a diver who died, trapped in this cave, 30 years ago.

BRIAN KAKUK: He's jammed up like he was trying to find his way out. He looks like a 1970s diving display. He used no guide line, no redundant systems.

NARRATOR: Nearby, the diver's head, with his dive mask still in place.

The presence of the body was known, but it's never been identified. And it's so deep, no one's ever attempted a recovery.

KENNY BROAD: It's really knowing your limits; that's what keeps you alive. And I think that's what has killed a lot of people in caves.

You don't know what's in there. You know no one has been in a lot of these places and that's kind of sucking you in. Our human nature is to push into places that we don't know about.

NARRATOR: And the blue holes of the Bahamas appeal to that curiosity with their hundreds of miles of unexplored passages.

The blue holes formed as long as 300,000 years ago, in the middle of the Ice Age, when ice caps expanded and sea levels dropped.

Then, sea level was over 400 feet lower, and the Bahamas sat high and dry.

Over time, rain carved holes in the limestone base of the Bahamas islands, creating caves. When sea levels rose again, the caves filled up. Ocean water settled at the bottom, fresh water accumulated at the top.

Eventually, the deep layer became completely devoid of oxygen, and that's what makes blue holes time capsules. Without oxygen, bones and other hints of the past are preserved. It's one of the main draws for cave explorers.

After over an hour, a cave-in and an encounter with a corpse, the shakedown dive ends, amid intense relief.

WES SKILES: That was really fun. Good god, what a hellhole.

KENNY BROAD: Those were the most intense 78 minutes I've spent in a long time.

NARRATOR: As cave divers often say: a successful dive is one you return from.

KENNY BROAD: You think you're home free, and all of a sudden the cave starts raining on you. We're laughing because we're here to laugh about it.

Let's go cry by the van.

NARRATOR: With their gear and fortitude tested, they're ready to take on a biological cold-case, thousands of years old.

Though the Bahamas are just 60 miles from the coast of Florida, they have only a tiny fraction of Florida's wildlife. There are no crocs or tortoises in the Bahamas, and no big predators of any kind. But it wasn't always this way.

David Steadman wants to know why. He's one of the world's top experts on island ecology. He suspects that the Bahamas was once a thriving Eden.

And he's here to find proof.

DAVID STEADMAN (University of Florida):We've underestimated the biological potential of islands. Islands used to be teaming with species. The trouble is they're more vulnerable on islands.

NARRATOR: For centuries, hurricanes have scoured these islands clean. On the surface, there's almost no fossil record of anything, so the best place to find bones may be straight down, in a blue hole named Saw Mill Sink on Abaco Island.

Its high sides give it the look of a natural animal trap.

The team thinks it may be a good place to hunt for the remains of animals who lived here in the past.

DAVID STEADMAN: Scoop up the sediment, scoop as deep as you can.

NARRATOR: Since Steadman is not a diver, the underwater team will be his eyes and hands, prospecting for bones in another very dangerous place.

WES SKILES: This is a very disorientating place, Nate, so ya'll watch each other.

DIVER: Stay on the line, stay on the line, stay on the line.

NARRATOR: Before they reach the bottom, where fossils would be, they face one more perilous obstacle: a thick layer of poison hydrogen sulfide. The naturally orange-tinted solution is actually the excrement of billions of tiny bacteria. They eat organic material, then excrete hydrogen sulfide—and another deadly barrier to blue hole exploration forms.

The divers can't protect themselves from this poison because it enters through their skin. Here, at about a hundred parts per million, hydrogen sulfide makes the divers' lips tingle and smells like rotten eggs. But prolonged exposure may cause brain damage, so they've got to kick through it fast. Once on the other side, they reach the oxygen-free zone. It's here that the bones of animals that fell into the blue hole should be preserved.

DAVID STEADMAN: We're dealing with, sort of, an aquatic tar pit. Animals get in there, and as they die, they sink down into the part of the water that has no oxygen in it. Because that water has no oxygen in it, there's nothing to support the microorganisms—the bacteria and fungus and things like that—that normally would decay bones.

NANCY ALBURY (National Museum of the Bahamas) : In Saw Mill Sink, fossils that are 4,000 years old appear better than a chicken bone that may have been thrown in your backyard the week before.

NARRATOR: Prospecting for fossils in a blue hole takes a delicate touch. Even gentle probing can stir up the silt, which could create a dangerous blackout.

As he probes, Brian makes a find.

It's a shell, the shape of some kind of tortoise.

The oxygen-free layer doesn't preserve skin or muscle, but it does maintain shell and bone in almost pristine condition.

About 20 feet away, he finds the bone from another creature. It's a piece of a backbone, but whose?

And then, up from the muck comes the answer, a foot-long skull, loaded with teeth. It's an ancient crocodile, so perfect, it looks like it's been kept in a museum. Large animals like these don't exist on the Bahamas today.

These finds are evidence that there may have been a lost world here.

With these prizes in hand, the team follows the line deeper into the cave.

In this part of Saw Mill Sink, the cave opens up. Thousands of years ago, when sea levels were lower, this soaring hall was dry and would have made a perfect home for birds.

Today, the divers can actually swim up to a ledge where birds might have roosted and start scouting for fossils.

On the ledge, they find a bowl-shaped roost. Below it, they set up a grid, surrounding a field of tiny bones of many different types. This suggests that the bird roosting here was an owl. The bones are the remains of what he was eating.

Since they can't digest bones, owls collect them in their crop and vomit them out in a clump called a pellet.

This is a fossil gold mine: the bones of lizards, snakes, bats and small birds, a census of life here thousands of years ago.

It will delight David Steadman, waiting for the divers to return.

BRIAN KAKUK: We got it.

DAVID STEADMAN: Yeah, I can't wait to open these up.

BRIAN KAKUK: There. There may be some associated bones in the sediment that's in the container there, too, so we'll be careful with that sediment.

NANCY ALBURY: Okay, got it. Oh, wow!

DAVID STEADMAN: It's complete, a complete tortoise.

NARRATOR: Steadman has a hunch there's a link between the tortoise and the crocodile.

DAVID STEADMAN: There's no way, once a tortoise would fall in here, that it's ever going to get out. There's no exit.

And you can imagine a crocodile go in to eat the tortoise. And after it ate the tortoise, the crocodile couldn't crawl out of this sink hole either.

NANCY ALBURY: This is so neat. No one has ever seen this animal before. We're the first ones that actually get to touch it and feel it and study it and know that there was something special going on here.

Can you imagine what this place once was? Having crocodiles running around here?

NARRATOR: When Steadman examines the tortoise shell, he makes an astonishing find. Bite marks from a crocodile prove his hunch was right.

DAVID STEADMAN: This is the sort of thing that makes the fossils from these blue holes so spectacular: to have this kind of evidence, not just of what this animal looked like, not just of what the crocodile looked like, but evidence of who is eating who.

NARRATOR: Then he turns to the finds from near the owl roost.

DAVID STEADMAN: This is, sort of, the sediment sample of my dreams.

NARRATOR: Never has owl vomit looked so good.

DAVID STEADMAN: All of a sudden, we have a number of complete skeletons to study. We're finding species that nobody knew existed before.

NARRATOR: With each bone he identifies, the picture of an ancient, wild Bahamas is starting to emerge.

DAVID STEADMAN: The three largest bones are the three major leg ­­­bones of a flightless rail, an un-described species of flightless rail, unknown to science.

NARRATOR: This new species joins the croc and tortoise, as the list of animals found in Saw Mill Sink grows: a lizard, a meadowlark and a burrowing owl, a Bahamian boa constrictor and a raptor, called a caracara. From just a single bag of sediment, Steadman identifies 35 species previously unknown here. Three are entirely new to science.

It's clear that centuries ago, the barren islands of the Bahamas weren't so barren. So what happened?

DAVID STEADMAN: The bones I've picked out so far, every one of them represents a species of bird that isn't on the island anymore. Based on just this tiny sample, it looks like we're on our way to documenting a pretty drastic change.

NARRATOR: About a thousand years ago, most of these animals mysteriously disappeared. A fossil trail extending back roughly 12,000 years suddenly goes cold. What killed these animals off?

In 1492, when Christopher Columbus first made landfall in this hemisphere, it was in the Bahamas. He encountered a people called the Lucayans.

The Lucayans descended from people that came from South America, around 800 B.C., and slowly migrated to the Bahamas.

At the time, Columbus wrote almost nothing about their rituals or traditions.

What little we know today suggests that they thought blue holes were sacred. They believed caves were the birthplace of humankind, and that when someone died, they should be put back inside.

In the early 1990s, in a blue hole called Sanctuary, on Andros Island, a diver found human remains that may have been related to the Lucayans' ritual burial.

Following up on that lead, the team will search for more remains, beginning at Sanctuary. They'll try to determine if there's a link between the Lucayans and the animal die-off that occurred about a thousand years ago.

WES SKILES: Our goal is to go down there, look for those identifying signs that there are—bones, skulls—hopefully, recover that and document it all.

NARRATOR: Sanctuary is protected by a series of natural barriers. For starters, it's 110 degrees with humidity to match.

KENNY BROAD: If you're looking for liquid weight loss, it's a great site. If you are looking to stay hydrated, it's not the best site.

NARRATOR: Underfoot, a type of jagged dead coral called needle rock, and throughout the scrub, a toxic plant named poisonwood, which they've marked with red tape. Poisonwood is like poison ivy on steroids.

Joining the team is Michael Pateman, who'll be the first Bahamian and the first archaeologist to dive this hole.

MICHAEL P. PATEMAN (National Museum of the Bahamas) : I have been looking forward to this since I started archaeology. It's, it's the mystery of the site. It's "What? What can I find?" I want to know what's here.

NARRATOR: Once on the line, the team begins their descent to the point in the cave where, 20 years ago, a diver found human remains.

They'll search in an area that's deep—260 feet. It's risky to go that far down, because the effects of pressure can kill a diver.

At sea level, the pressure exerted on a person is called one atmosphere. That's the weight of the column of air above each individual, extending up to space. It's about 15 pounds per square inch, pressure we live with every day. But at the team's target site, 260 feet down, they'll be under nearly nine atmospheres of pressure. That's almost 135 pounds per square inch.

Under pressure, the gases in air, mostly oxygen and nitrogen, enter the diver's bloodstream faster. Our bodies metabolize oxygen but not nitrogen, so, as they descend, nitrogen bubbles build up in the divers' bloodstream. Go too deep and that nitrogen buildup can cause narcosis. That's like being drunk underwater.

And nitrogen kills another way. If the divers rise too quickly, the nitrogen bubbles in their blood will expand, blocking blood vessels, starving cells of oxygen. That can cause a fatal sickness, known as the bends.

BRIAN KAKUK: If we come up too fast and the bubbles are at a larger size, then they start to tear apart tissues, uh, nerve endings, things like that.

NARRATOR: For now, the team is still on their way down to the bone discovery site. At about 40 feet, they pass through a blurry layer called the halocline. This is where fresh water above meets saltwater below. As salt and fresh water meet, a chemical reaction occurs, corrosive enough to eat away at the cave wall.

The halocline usually forms just below sea level. So, over time, as sea levels rose, the halocline carved out the numerous side passages from the main cave. At 260 feet, they're just above the slope where human bones were found and face another potential danger.

The surrounding rocks indicate there's been a massive cave-in.

KENNY BROAD: That kind of stuff goes on out of sight all the time, where you have chunks coming down. We're swimming over boulders the size of houses, and you know they came from the ceiling.

NARRATOR: They know it will happen again; they just don't know when. It's one more detail that makes working in a blue hole more dangerous than any other type of marine archaeology.

KENNY BROAD: Archaeology in open water, if you have a problem, you can do immediate assent up to the surface. If you need to haul material up, it goes straight up to the surface. You can't do any of that in an underwater cave.

NARRATOR: In spite of the challenges, it pays off with, first, a human jawbone and then a skull.

For Bahamian archaeologist Michael Pateman, the find leaves its mark.

MICHAEL PATEMAN: To finally go in the site and to see the site, I thought, "Wow, I could spend hours in here." And to have been the first Bahamian to dive in that blue hole was just...it's still not sunk in completely yet.

NARRATOR: By the end of the dive, the team has recovered pieces of three skulls. But critical questions remain: are they connected to the catastrophic die-off of animals here? And could these skulls belong to the Lucayans, the same people Columbus wrote about?

Pateman identifies a deformity in the skulls that seems to confirm their identity.

MICHAEL PATEMAN: When they were children, the Lucayans bound the foreheads and the backs with boards and then wrapped them. And so that would create this, what you see here, this conical shape.

NARRATOR: It's a characteristic found only in cultures like that of the Lucayans.

And when the skulls are radiocarbon dated, there's an even bigger revelation. The skulls found in the blue holes date from about 800 years ago.

By then, the Lucayans were already settling in the Bahamas and having an impact. And, the animal die-off that David Steadman identified started about a thousand years ago.

The dates are close enough that it's likely there's a link between the Lucayans and the deaths of the animals here.

DAVID STEADMAN: We have great fossils, from about 12,000 years ago up to about when the first people arrived.

So when people show up, along with non-native mammals that they bring—whether it's rats, cats, dogs, pigs, things like that—these island species are very poorly adapted to these new super-predators.

On islands across the world, the most vulnerable species have been wiped out. They're extinct.

NARRATOR: By exploring the time capsules inside blue holes, these divers have discovered a lost Eden-that-was and showed how humanity changed the Bahamas forever.

But bones aren't the only prizes preserved in blue holes. Locked in spires like these are secrets about the planet's history, secrets that can only be found in blue holes. So, now, the team heads for blue holes famous for some of the most spectacular spires in the Bahamas. They'll dive for them in a pair of blue holes on Abaco Island.

To find and collect the right type of spire, known as a stalagmite, dive leader Brian Kakuk and anthropologist Kenny Broad will face their most technically demanding dive.

GORDY: How deep are we, right here, man?

KENNY BROAD: Well, right now, we're at the surface. You're actually above the surface, but this hole goes 320 feet.

NARRATOR: Cutting through the solid rock at the base of a stalagmite is going to take a lot of time.

The maximum they can stay down with two tanks of regular air is less than two hours. But they need almost three times that for this dive. So they're using a device like astronauts use during a spacewalk, a machine called a re-breather. Re-breathers are computer-controlled devices that let you breathe your own air over and over again.

JILL HEINRETH (Technical Diving Specialist): Re-breathers allow us to scrub out carbon dioxide, the bad things that we don't want to re-breathe.

So these little tanks will allow us to do a very long, deep penetration of a cave.

NARRATOR: Once carbon dioxide is scrubbed out, sensors check the amount of oxygen in the air, adding more if necessary.

KENNY BROAD: This is the canister that filters the air that we exhale. It takes out carbon dioxide, cleans it out. And then this is the magical brain that tells how much of other types of gases—whether it's air, helium or, most important, oxygen—how much to put back into the breathing system. It's very important.

Be nice to me. Be nice to me.

NARRATOR: Re-breathers are especially good for cave diving, because they release few bubbles, which can dislodge ceiling debris, creating blackouts. But they do have a deadly downside.

JILL HEINRETH: There's as many as one out of 10 re-breathers sold that people have died on. I mean, this is real serious business. It's a very complex piece of equipment. It needs to be prepared properly. You've got to be on your toes, and you need to be monitoring it constantly.

NARRATOR: Before the divers use a re-breather, they'll go through a checklist more than 40 items long.

JILL HEINRETH: Every time I prepare my re-breather, I think about the friends that I've lost and buried over the years, and, um, that gives me extra pause and extra care in my preparations.

KENNY BROAD: And there's not the same warning signals that things are going wrong. With the re-breather, you don't get the sign. You're breathing fine, but you're not sure exactly what gas is in your mixture.

BRIAN KAKUK: The oxygen content of that mixture might not actually be enough to sustain consciousness, so you never feel it coming, it's just lights out.

NARRATOR: On this dive, re-breathers offer Kenny and Brian a tempting but sobering bargain: nearly unlimited time to work in the deep, at a significant risk of dying on the job. Avoiding that requires intense concentration and, for Kenny, that's the appeal.

KENNY BROAD: It's how I can disconnect. I feel like all day I'm attacked by my cell phone, I'm attacked by my e-mail; it's the only place in the world where that has to be shut out. It's the only time you can have 100 percent concentration, because it's a life or death endeavor.

NARRATOR: At 150 feet, Brian and Kenny transit a series of rooms girded by stalagmites. These mineral formations grow from the ground up. Some in this cave took more than 200,000 years to form.

It started in a dry cave, when water percolated through the ceiling and fell in different spots. Stalagmites are built by the slow accumulation of the minerals and sediments contained in the water. So these beautiful columns are more than geological ornaments. Locked inside is a biography of Earth and its atmosphere.

The stalagmites Brian and Kenny are harvesting may help answer critical questions about the history of Earth's climate. That's what geo-chemist Peter Swart wants to know about.

PETER K. SWART (University of Miami) : Stalagmites give us a clue to what paleoclimate was many thousands or hundreds of thousands of years ago.

NARRATOR: Much of what we know about climate history comes from two sources: sediment samples from the ocean floor and cores taken from glaciers and ice sheets. But stalagmites from these caves have one major advantage in recording climate history: their location.

The Bahamas lie beside the Atlantic Gulf Stream. For millions of years, this current has carried warm, salty waters north, where they cool and sink. That drives ocean circulation, also called the Atlantic gyre, which shapes the climate.

PETER SWART: Stalagmites record changes, right in the sub-tropical Atlantic gyre. So, here, we're really at the heart of the action. We haven't had that kind of opportunity to see the changes which occur in this region before now.

NARRATOR: So the stalagmite that Brian and Kenny have cut could provide a gold mine of global climate history. Using that history, Swart may be able to answer a critical question. While most scientists believe that Earth's climate is changing, they are struggling to figure out how fast. This stalagmite could help answer the question.

To decode it, Swart first has to cut it lengthwise with a diamond saw.

Inside, it displays regular bands of growth, like the rings in a tree. Embedded in each band is the climate biography of a specific period.

With analysis, Swart can describe how much it rained, what chemicals were in the rain, soil and air, even the temperature. Examination of the stalagmite shows evidence of more than five major climate changes over the last 80,000 years. This corroborates findings in samples taken from ice cores.

But then he notices two curious details: first, the way the bands formed suggests that at least one of these climate episodes came on very abruptly.

At the end of the last Ice Age, about 11,000 years ago, conditions changed radically, from dry to wet, within 50 years. This change was probably accompanied by a rise in temperature and sea level.

And Swart notices something else: some of these climate events are preceded by a mysterious dark band. When he samples the dark bands, he finds iron.

So is there some link between iron and climate change? How could that be? The Bahamas are made of coral; there's no iron anywhere.

In the blue hole where they cut the stalagmite, Brian and Kenny noticed something that might help solve the mystery. In the cave wall, they found a layer of red sediment. The color is the key here: the red in the dust means it's loaded with iron. And that means that thousands of years ago, a thick layer of iron-rich red dust covered the island's surface.

Repeated rains washed it through the rock, leaving a bright red band.

So how did the red dust get here? One theory suggests it came from the Sahara Desert, some 4,000 miles away.

During times of extreme drought, towering dust storms gather in the Sahara, pushing dust high into the atmosphere, where it's carried across the Atlantic.

When Swart analyzes the iron in the stalagmite, he confirms that it's made of Sahara dust, though its red color has been washed out after thousands of years.

And in the dark band, he isn't finding traces of iron, but significant concentrations.

PETER SWART: Now, the iron was very low concentrations, with the exception of this boundary here.

NARRATOR: The areas with the highest concentration of iron correspond almost exactly to the places on the stalagmite where the chemical composition indicates a period of major climate change.

That probably means a major Sahara dust event came right before each change, when temperatures and sea levels rose.

The fact that Saharan dust storms happen with greater frequency today is raising concerns that history could be repeating itself.

PETER SWART: Now, we know, for the last 40, 50 years, there's been a major drought in Africa. And that has seen an input in the amount of dust which is coming from the Sahara region to the Bahamas.

NARRATOR: It's estimated that over the past five decades, the Sahara has seen a ten-fold increase in large-scale dust storms. If we are witnessing the beginning of a major climate change, it could happen fast, just as it happened in the past, maybe in as little as a lifetime.

PETER SWART: We don't worry too much about climate change, because it's something that's going to happen "after I'm dead." But, in actual fact, some of the records that we've been looking at, we see tremendous changes in a matter of decades. And so, when climate changes that fast, obviously it would have tremendous implications for the present-day society.

NARRATOR: Swart's findings are preliminary, but they do suggest that climate change in the past happened faster than anyone imagined. If such change occurred today, immediate concern would be for the millions of people in areas most affected by sea level rise—island nations and coastal regions throughout the world.

For the expedition team, climate change could pose an immediate threat to the future of blue holes. There's probably no way to keep rising seas from pouring into them, ruining their delicate chemistry.

KENNY BROAD: Blue holes are unique repositories of scientific knowledge. Sea level rise associated with global warming will change that water chemistry, and we are going to lose a lot of valuable information very quickly.

DAVID STEADMAN: We would lose most or all of the fossil record. It would be the equivalent of burning a library.

BRIAN KAKUK: Eventually, the sea will rise, and once that happens, all these artifacts will start to deteriorate. So we really feel like we need to get as much information out of these places as we can, right now.

NARRATOR: Losing blue holes is hard to contemplate for the explorers who have touched the depths of this lethal and alluring lost world.

WES SKILES: You know, I'm not sure I would know how to go on in life, if I wasn't able to go to my church under water. We go down in these places and we're cleansed. When we come up from a dive, the pressure, the challenges, the difficulties, the conflicts, they're gone. We come up from a dive and it is like you're reborn.