Deep-sea exploration, of course, is an inherently dangerous pursuit.

Venturing nearly seven miles down into the Challenger Deep, for example, means  surviving perhaps the most extreme, forbidding environment on the planet. The pressure is 1,000 times that experienced at sea level, the rough equivalent of having 50 jumbo jets piled atop the submersible, and the temperature varies from just above freezing to hydrothermal vents where the water can reach nearly 400 degrees Celsius. Visibility is hindered both by the darkness–at below 1,000 meters in depth, the waters are pitch black–and superfine sediments that are stirred from the bottom by a submersible.

These qualities subject submersible vehicles and their gadgetry to almost unimaginable degrees of stress that increases the risk of breakdown, and allows precious little margin for error.  In this article, filmmaker and deep-sea explorer James Cameron lists the potential hazards that deep-sea explorers face, ranging from entanglement in a communications cable to the risk of getting too close to a hydrothermal vent and suffering a melted viewport.  As this Australian newspaper story on adventurer Sir Richard Branson’s plan to visit the Challenger Deep notes, at that depth, a crack in a submersible’s hull “could cause it to implode and be crushed like a paper cup.

In addition to the ever-present danger, deep-sea explorers also must learn to cope with varying degrees of discomfort. As University of Alabama geologist Paul Aharon explains in this Sciencebase article, venturing into the deep ocean in the Alvin, a vehicle operated by the Scripps Institution of Oceanography, meant spending hours crammed into a tight space with oxygen tanks and other equipment, and being unable to move one’s legs for long periods, which only accentuates the freezer-like chill.  (“I almost got hypothermia because I forgot to take long pants with me,” he noted.) Additionally, Aharon and his two crew mates had to breathe air that had been altered to reduce the amount of oxygen and increase the CO2 level, as a precaution against a spark causing a catastrophic explosion. The tradeoff: headaches, memory lapses, and slowdowns in brain functions.

Despite all those risks, however, deep-sea explorers maintain an impressive safety record. The most memorable mishap in the annals occurred during the bathysphere Trieste’s history-making descent into the Challenger Deep in 1960, when a non-essential window cracked. “There was this great big bang at 30,000 feet,” recalls retired Navy submarine officer Don Walsh, who piloted the craft. “But we checked all our instruments and looked out through the view ports nothing seemed to be wrong, so we kept going. It turns out that it was a window that wasn’t a pressure boundary–it was subjected to pressure on both sides. So it was no big deal. But everybody loves that incident–if I had a nickel for every time I’ve been asked about it, I’d be a wealthy man.”

Indeed, the worst undersea accidents have been suffered by military submarines at far shallower depths than the Challenger Deep. One of the worst, as this National Geographic article details, was the April 1963 loss of the nuclear attack submarine Thresher, which was conducting deep-dive trials off the coast of Cape Cod, Massachusetts, apparently when a piping joint in a sea water system in the engine room gave way. The resulting spray shorted out electronics and forced an automatic shutdown of the nuclear reactor, which meant that the sub lacked the power to keep from sinking from 1,300 feet to 8,400 feet. The drastic increase in pressure was more than what the sub’s hull could withstand. “As they sank, the men aboard would have heard piping and fittings giving way,” the article notes. “They would have listened as the ship’s hull creaked and groaned, until it finally, deafeningly gave way to massive water pressure. All 129 men aboard were likely killed within a matter of seconds.” Radio operators on the U.S. Navy submarine rescue ship Skylark, which was in contact with the Thresher, heard only fragmented, garbled radio messages, followed by silence, and then the horrifying sounds of the submarine itself breaking apart and imploding.

In 2000, a comparable tragedy occurred in the icy waters of the Barents Sea, when the giant Russian nuclear submarine Kursk sank, trapping the 118 men aboard. The incident, documented in investigative journalist Ramsey Flynn’s book Cry from the Deep, was all the more horrific because 23 sailors managed to survive for a time by seeking refuge in the rearmost compartment of the submarine, but the Russian navy failed to come to their rescue. Flynn concluded that the accident was caused by the explosion of a hydrogen peroxide-fueled torpedo, which caused a fire that triggered more ordinance to explode.

And to keep the risks in perspective, surface explorers of the oceans in previous centuries faced dangers as well–and suffered far more from them. In his 2004 book Over the Edge of the World: Magellan’s Terrifying Circumnavigation of the Globe, historian Laurence Bergreen notes when Ferdinand Magellan’s expedition in search of the Spice Islands returned to Spain in 1522, only one of five ships and just 18 of the 260-man crew made it back.

Some, in fact, suggest that our extreme aversion to risk has hindered deep-sea exploration. Sylvia Earle, founder and Chair of Deep Ocean Exploration and Research, Inc., points out in an article on the NASA website that only 5 percent of the oceans–the home to 97 percent of life on Earth–has been explored. “What really is at risk is our future,” she argues.

Don’t miss the premiere of a new half-hour special chronicling Cameron’s historic one-man dive, James Cameron: Voyage to the Bottom of the Earth, Sunday, April 29, at 9PM and 9:30PM et/pt, only on the National Geographic Channel. For more information, visit http://deepseachallenge.com/