For most people, hearing the word "shark" conjures up images of violent predators in the Oceanic underworld. It's the stuff of nightmares, unless you're like Enric Sala, who revels when swimming in shark-infested waters.
"Unfortunately, when you have a camera, the sharks are never close enough," said Sala, a National Geographic ocean explorer who spent three weeks swept up in a tide of sharks as part of a marine-research expedition.
"You want them to be closer. And we can get really, really close to the animals to be able to study their behavior. ...These sharks are so afraid of humans that when they get really close and see the bubbles, they swim away."
Sala and his team traveled in September to Costa Rica's Cocos Island, known to some as "Shark Island" for its high concentration of sharks, to see if a bold experiment in survival was actually working.
More than 30 years ago, the waters around the island were turned in to a national park and declared off limits to any kind of fishing. Researchers hoped to discover if the vibrant sea life would return, and if the island could become a model for conservation.
"Nightline" met up with Sala, 41, at Long Island, N.Y.'s Atlantis Marine World to talk about his expedition and upcoming documentary "Shark Island," which will air on the National Geographic Wild channel Monday.
Sala led the expedition with world-famous oceanographer Sylvia Earle.
"She has spent thousands of hours underwater," he said of his colleague. "She was key in living the deep part of our expedition because we were studying the sharks, but we were also studying some deep-sea mounds where nobody had been before."
Las Gemelas seamounds are an entire underwater mountain range with peaks that top out at 500 feet below the water's surface. Sala compared it to landing on the dark side of the moon.
There, diving down using underwater submarines, they found a barrage of sea life, but there was also something disturbing in the midst: fishing lines.
Sharks have been poached from these protected waters by fishermen, eager to ship them off to Asia, where consumption of shark-fin soup is a symbol of wealth and prestige.
"It's a very popular dish in China, for example. And now it's become a sign of social status," Sala said. "Most of the Chinese weddings have to have shark-fin soup."
Heavy demand explains what the crew found days later in the protected waters, baited lines more evidence of poachers.
The team saw a 6-foot-long yellowfin tuna hooked for hours. By the time they were able to free the battered giant tuna, it was too late. It slowly sank to the bottom of the ocean, dead from the exhausting struggle.
A green sea turtle also fell victim to fishermen that day.
"The turtle fought really hard and the turtle doesn't know that you want to help her and still fights harder when you try to unhook her from the line," Sala said. "But, fortunately, we were able to release and that was the fastest turtle I've seen, it went down and disappeared into the safety of the world."
Poaching is still a huge problem on Shark Island. In the past five years alone, more than 450 sharks, a thousand yellowfin tuna, and turtles and dolphins by the dozens have been found hooked to fishing lines in the park.