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Published May 3, 2011

An Australian team, led by National Geographic Society/Waitt Foundation grantee Richard Harris, has made a record-breaking dive in what may be the world's deepest coldwater cave. But the explorers still haven't reached the end of this New Zealand cave, thought to lead to the mysterious source of the Pearse River.

© 2011 National Geographic; Videographer: Richard Harris

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UNEDITED TRANSCRIPT

Divers cautiously descend in a dangerous and uncharted underground "river"searching for its mysterious source. On their way, they’ll look for new life forms living deep within the cave…

Nowhere else in the world are sport divers going so deep in such cold waters.

In a remote part of New Zealand’s South Island, the Pearse River emerges out of a mountain beneath a cliff. And after years of exploration and research, its exact origin is still unknown.

The river emerges from a deep underground cave, which might be the deepest coldwater cave in the world.

The cave does not go straight down from the surface entrance, but meanders a bit before taking a mostly sideways path into the mountain.

It’s a challenging environment. The depths found in the cave make diving problematic, and the cave itself is very dark. The current in the river is strong, and the water is just above freezing.

Because of the extreme conditions, divers have been unable to reach the cave’s source. One diver lost his life attempting to explore the main shaft in 1995.

The most recent expedition, funded in part by National Geographic, consists of a a team of six Australian cave divers led by diving physician Richard Harris. Earlier this year, a member of that team accomplished what no previous explorer had: he reached 194 meters in depth, or about 636 feet.

The Resurgence is in a remote location, and getting 5 tons of equipment there requires helicopter delivery. This trip required a nearly 3-week long encampment.

But it takes special diving skills to go so deep, and for so long.

The glacial waters are about 43 degrees Fahrenheit, and divers must swim against the flow. They must also swim at a measured pace, because over exertion can lead to carbon dioxide poisoning, which can lead to death. Because of the great depths at which they’re diving, they must slowly—over a period of hours—return to the surface. Failure to do so could result in decompression sickness, also known as “the bends,” which can be fatal. The cold water alone makes resurfacing safely problematic.

To overcome these obstacles, the team constructed and placed habitats at four depths in the cave. Here, divers can escape the cold water and sit in a trapped bubble of air. They can eat here, and drink, while they wait many hours for dissolved gases to be safely released from their bodies.

Even with these safety measures in place, this kind of diving is still dangerous.

During his record-breaking dive, experienced diver Craig Challen realized his breathing had become erratic.

CRAIG CHALLEN, TECHNICAL DIVER AND CAVE EXPLORER: “I noticed going down, and I just started breathing really heavily. It’s really like a self-perpetuating thing, you know. You try and relax and chill, and you just can’t.”

It’s likely he had just enough time to reach a safer depth, before the carbon dioxide levels could have led to unconsciousness.

CRAIG CHALLEN, TECHNICAL DIVER AND CAVE EXPLORER:“There’s definitely a 200-meter dive there for someone that wants it. (laughs).”

Challen and the team know a record-breaking attempt is not for the faint of heart.

The mystery is part of the reason explorers keep coming back.

Dye tracing, so far, has been inconclusive in finding a source of this river.

Freshwater animals found in the cold aquifer, like this possible new species of amphipod, also offer scientists a unique look into a little understood, and little studied habitat.

The team hopes to return to the Pearse Resurgence for another exploration dive in 2012.

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