Exploring hidden cenotes and encountering the deadliest sharks

Meet Jonathan Bird, one of the world's top underwater nature cinematographers. Held on assignment for all the major networks, he is an Emmy Award winning authority on the underwater world. In freshwater or salt, reefs, wrecks or caves, Jonathan documents the world beneath the waves. Welcome to the Blue World. I'm here in the jungles of the Yucatan, which is not the kind of place you'd normally expect to find an underwater kind of guy like me. But flowing beneath these forests are incredible underground rivers. And in places, holes in the forest floor like this one provide access to an amazing unseen world. The holes in the ground are called cenotes from a Mayan word meaning sacred well. But not all cenotes look like a hole in the ground. Some appear at first to be ponds, yet they all share one common trait— their openings to underwater caves and caverns. Mexico's Riviera Maya in the Yucatan Peninsula is a vast rainforest filled with hundreds of cenotes. There are thousands of miles of underground rivers here. My adventure begins first thing in the morning at H2O Pro Diving in Tulum, where we load our gear into a van to begin the journey out into the middle of nowhere, towards the cenote. My guide Marco Wagner is an experienced cave diving instructor who will keep me out of trouble with a safe introduction to the sport of cenote diving. So I can hardly wait to see this thing. Unless you are specially certified for cave diving, you should only dive with a local guide who knows the cenote well. Today we're diving a famous cenote called Calavera. Calavera means translated skull. And you see the big mouse and you see the small eyes. These are all the small eyes where we see in the dive where the light beams are coming in and they look like disco lasers. Wow, this is really way out in the jungle. How do you think anybody found this place? I mean, they get explored by the owners of the land also itself. They discover cenotes because they have huge pieces, huge chunks of land. And actually when they chop the trees down and they make trails, they found the cenote also from... Cenotes are here for the ancient Mayans also, which were like sacred wells for them. That's where the cenote is coming from the word. It's coming from the Mayan word cenote. That means sacred well. So the Mayans, this was a really important source of water or... They also used the groundwater as well, but also for sacrificing for the gods. Some cenotes were used. But still the groundwater also still comes from this underground river that we have here in the peninsula. Wow, that's cool. It's deep too. So we're going to have to jump down. How far is that? That is about 2 meters 50, 3 meters more or less. Echo! Oh yeah, that's going to be fun. Now it's time to begin our dive. We grab our gear from the van and suit up near the road. Marco is wearing double tanks and all kinds of cool technical cave diving gear. Me? I'm just wearing my normal scuba gear and a few extra flashlights. The walk-in was plenty long and hot enough when I wasn't wearing my dive gear. That water is going to feel so good. Marco gives me some last words of advice before I take the plunge. I think I'm more worried about the 10 foot drop into the water than the dive itself. Cliff diving! Woohoo! Yeah! That's great! The water is extremely clear. I adjust my camera lights and follow Marco down below the earth. As we enter the cenote's cavern, I realize that I'm truly heading into another world. This is not my area of expertise. I follow Marco carefully and pay attention. We've brought some humongous lights to help illuminate the cavern for photography, and they really help show what it looks like. It's beautiful! The tunnels were formed by water rushing through and dissolving the soft limestone. But the intricate decorations hanging from the ceiling called stalactites can only form in a dry cave. Thousands of years ago, during the last ice age, the water levels were lower and these very caves were bone dry. That's when these impressive stalactites were formed. On a visit to a dry cave, I can observe the actual growth of stalactites. But it's pretty slow. It works like this. When it rains up in the rainforest, the water percolates through the limestone, and along the way it dissolves a little bit of the limestone. Then, it drips from the roof of the cave. And over thousands of years, each drip leaves a little bit of limestone behind, forming an icicle-like structure called a stalactite. They get longer and longer and longer with time. But the water continues to drip, and eventually it hits the floor of the cave, forming a little bump. And that bump gets bigger and taller and taller, forming sort of like an upside down icicle called a stalagmite. If you wait long enough, the two of them keep getting longer and longer until they connect into what is called a column. At the end of the ice age, the sea levels rose. And so too did the groundwater levels in the Yucatan. Soon, these huge subterranean caverns filled with water, leaving the stalactites frozen in time. In some places, the caverns have huge air pockets where the water doesn't go all the way to the top. Here we can surface for a breath and take a look around. Some of the cenotes can even be explored by snorkelers. Good buoyancy control in the cavern is essential. The limestone formations are incredibly beautiful, yet fragile sculptures. The formations took thousands of years to form, but a single careless fin kick could destroy them in seconds. So we must kick gently and stay off the bottom. The floor of the cenote is covered in super fine silt. Swimming too close to the bottom would stir it up and reduce visibility to zero, which could be very dangerous. As I follow Marco deeper, the water starts to get weird and blurry looking. Many of these underground rivers make their way to the ocean. At some point along their journey, the water transitions from fresh to saltwater. Saltwater and freshwater have different densities. The interface between the two different densities can cause an optical condition called a halocline, where the water appears blurry. It's a weird, swirly, foggy haze. I follow Marco down below the halocline and into the saltwater. I can taste the salt on my lips, and the temperature is about 5 degrees warmer. Soon we reach the Swiss cheese room. Down here, a chemical reaction with the saltwater eats away some of the limestone, creating thousands of holes in the rock and the unique Swiss cheese appearance. It's a completely different kind of formation than I saw in the freshwater part of the cavern. As we swim into the far reaches of the cenote, we eventually reach a point where we can no longer see sunlight from the opening. This has now become true cave diving, a much more dangerous activity. Here we find a sign warning divers to stop and go no further. People have become lost and drowned in these vast labyrinths. This is as far as we go. Turning around, we head back out towards the entrance. Just before we reach the light of day, we find some Mayan pottery on the bottom, probably thrown into the cenote as an offering to the gods. The Mayans believed that the cenotes were pathways to the underworld. Offerings of gold, food, pottery and even humans were made to the gods through the cenotes. With my scuba tank getting low, the time has come to leave the cenote, so I head back to the surface. In the shallow water just below the jungle, tiny fish come over to check me out. Some of them nibble on my hair and ears. They must be really hungry if they think I look like lunch. Wow, I can't believe all the cool things I saw in this cenote. Who would have thought that there's so much amazing stuff hiding under the floor of the forest? And even though I'm done with my diving for the day, I think it's time for one more dip. Most fish are completely harmless, but there are a few species of extremely venomous fish that can sting. In many cases, this venom is so powerful that it can actually kill humans. The most dangerous fish in the world live on Pacific coral reefs. Let's go take a look. Heading down, I begin my search for venomous fish. Most are well camouflaged and hard to find. With much assistance from an experienced dive master, I find a stonefish, well hidden in the reef. It can change color to match its surroundings. Unless you know exactly where to look, you will never see a stonefish until it moves. Incidentally, touching a stonefish like this is dangerous. The reef has a few dangerous characters, and the stonefish is by far one of the most dangerous. The expertly camouflaged stonefish is the deadliest fish in the world. Its dorsal spines contain a highly toxic venom. Its body is shaped like a blob, and it hobbles more than swims. Sometimes stonefish bury themselves in the sand. They are ambush predators, holding perfectly still for hours, waiting for something bite-sized to swim by. The stonefish has a sting so powerful it can kill a grown man, and the pain is incredibly severe. The stonefish has a slightly less toxic relative called a scorpionfish. It can still sting humans, but it's rarely fatal. It's just really painful. A small group of damselfish are hovering near the reef. But nearby, watching and not moving a muscle, is a scorpionfish. His perfect camouflage is the result of skin that can change color to match the surroundings. The only thing that gives him away is the slight movement of his gills as he breathes. On top of his body, his dorsal fin contains the venomous spines—the armor to back up the camouflage. He's watching the damselfish, who are blissfully unaware of his presence. Soon, a damselfish comes too close. Not far away, I find a lionfish hiding in the coral. If those needles all over the fish look sharp, I can assure you it's because they are. Not only that, each one is like an independent syringe, ready to inject venom into any animal foolish enough to try to eat a lionfish. Almost nothing can prey on these fish once they are fully developed. During the day, they generally don't do much, but at night, things change. A small fish on the sand is blissfully unaware of the predator emerging from the darkness. Like its namesake, the lionfish pounces on prey and easily consumes its victim in a single gulp. Small fish don't stand a chance. Not far away from the reef, an area of rubble and sand provides a habitat for a well-camouflaged predator. The spiny devilfish is almost invisible if it doesn't move. But the hunting has not been so good here, so it's off to a better spot. The devilfish crawls along the bottom using modified pectoral fin spines as feet. The spiny devilfish sees a potential snack. Unfortunately, a line-cheeked wrasse saw it too. The devilfish is not pleased. Not far away, a scorpionfish is doing its best to look like a rock. A damselfish barely escapes, but not without injury. The scorpionfish goes hungry, but the spiny devilfish caught some luck. The injured damselfish has taken a turn for the worse. Although the stonefish, scorpionfish and lionfish are the most venomous fish in the ocean, they actually account for very few injuries to people. The fact is that if you leave them alone, they will leave you alone too. Many more divers are injured by sea urchins, jellyfish and crown-of-thorns sea stars than by venomous fish. But that's a story for another day. Don't go away! Jonathan is going after one of the coolest sharks around—the Greenland shark. We often think of sharks as creatures of the tropics, living in clear, warm waters near coral reefs, or perhaps prowling the shallows near a beach. But sharks live all over the world, in all of the oceans, from the tropics to the Arctic, from the shallows to the deep sea. This is one such animal, the Greenland shark, named after the place where it was first seen. This massive carnivore lives in the coldest water on Earth, the icy depths of the Arctic Ocean. But you don't need to go all the way to the Arctic to see one of these sharks. In recent years, divers have been getting surprise visits by Greenland sharks right here in the St. Lawrence. In the remote Canadian town of Baie-Comeau in Quebec, Greenland sharks come into shallow water during the summer. They enter a narrow, deep bay where the surface is darkened by tannins from river runoff. The visibility is not very good. I'm going out to see if I can dive with one of these monster sharks in the St. Lawrence. My guide to diving with the Greenland sharks is Sylvain Saurois, a local dive instructor and dive boat operator. He's been diving with the Greenland sharks for many years. So, Sylvain, how do we get the sharks to come over to us underwater? Do we have some bait or some chum or something? We're not going to use some bait. Those sharks are curious. They are predators, they want to know what invaded their territory, so they are going to come to see us. Wait a minute. You're telling me that they're just going to come over out of curiosity? Yes. We're strangers in their territory, so they're going to investigate us, turn around, and go back to their daily routine. Wow. Well, this I got to see. Let's go diving. Let's go diving. Drysuits take diving to another level of complexity. We begin suiting up to dive in the chilly water of the St. Lawrence. I need a drysuit to stay warm because the water rarely goes above 40 degrees. That's cold. I jump into the water, grab my camera, and head towards the bottom. But something is wrong. The water is not cold. It's nearly 60 degrees. Sylvain starts banging a pair of rocks together. He believes this will attract the sharks because they are curious about the sound. I see a kind of fish called sand lance, but no sharks. So this time, no luck Jonathan? Nothing. We went down, we sat at about 65-70 feet for an hour, and we looked all directions into the darkness. And I do mean darkness. And we saw nothing. I'm not one to give up easily, so we head out the next day to try again. Well, it's the second day of the trip. So far, four dives, zero sharks. Needless to say, shark diving is not always predictable. I enter the water again with my fingers crossed. We head to the bottom, engulfed in near darkness. The bottom is covered in beautiful anemones. And a crab scurries away from my lights. Sylvain does his best to attract the sharks, clicking his rocks together. But nothing shows up. The water is just too warm. 55 degrees at the bottom. Another dive with no sharks. Normally I'd be thrilled with water that's 20 degrees too warm. I mean, who doesn't love warm water? But these are Arctic sharks. They like really cold water. So if I want to find any sharks, I've got to find some cold water. And that probably means we're going to have to dive deep. The next day, we move the boat a little further from shore. Down on the bottom, it's so dark my camera can barely produce an image of Sylvain in the twilight at 110 feet. But good news! The water is cold at this depth. We only wait a few minutes and then, out of the darkness comes a big shape. It's a Greenland shark! I can't believe it! Just like Sylvain said, it's coming over to give me a look. No chum, no bait, just pure curiosity. What an odd-looking shark with a big, round body and a small mouth filled with rows of tiny, razor-sharp teeth. Most of my encounters with sharks are in warm, clear water. It's so weird to be swimming with a huge shark like this in cold, murky water. This shark is more than 10 feet long, but it's only a small one. Greenland sharks reach a massive 21 feet, making them among the largest carnivorous sharks in the world. Yet studies suggest they grow extremely slowly—less than an inch a year. So this 10-footer may be 100 years old. And that's nothing! Researchers think the Greenland shark may live to be 200 years old. Even though it's said that these sharks don't see well, this one appears to be watching me, his eye following my every move. The shark stays with me for 15 minutes, swimming slowly and not presenting any threat to me at all. He just seems curious. But soon he picks up the pace and I can no longer keep up. He swims into the gloom and my dive is over because I'm low on air. Sylvain and I head back to the surface. Finally seen a Greenland shark! We decided to go a little deeper, 110 feet, just so we could get into that colder water. And when we got down there, it was freezing cold and then out of the darkness came the big, huge Greenland shark. It was incredible! On the way back to the dock, I still can't believe what an amazing encounter I just had. Sometimes, all the effort is worth it in the end.