Then there's the helium they have to breathe to survive at such depths. Their voices are so high support crews need to use a special recording device to translate.
What's the point of this bizarre subterranean life? Coming up with a way to save drinking water for New York City, which is losing the equivalent of a small lake every day in an enormous, aging, leaky tunnel.
About half the city's water supply passes through the tunnel from upstate reservoirs. Of the hundreds of millions of gallons that flow there every day, some 10 million to 36 million escape from cracks in a 45-mile stretch. Not only is it a waste, the leaks create sinkholes and other problems at the surface.
The city Department of Environmental Protection, which oversees the water supply, has been aware of the leaks for decades, but repair work is complicated and takes careful and extensive planning, officials said.
The city began sending divers down to the tunnel in mid-February to gather data that will be used to develop a plan for repairs. The dive work is the first part of an early stage that will determine the best way to fix the tunnel. The project costs about $240 million and will take five years.
"There's not a ticking clock on this, but it's important to fix because this is 50 percent of the water supply," said Emily Lloyd, commissioner of the Department of Environmental Protection.
The city provided The Associated Press access to the project this week, offering a glimpse of the work performed by the divers.
The endeavor requires six men to live in the specialized chamber - a tube the size of a mobile home
- for three weeks to get acclimated to the change in pressure below ground. They breathe a mixture of oxygen and helium, because the nitrogen in regular air is too heavy at 600 feet and their lungs could not handle the pressure.
"It's cold, dark and very quiet down there. Beautifully quiet," said Tiffany Cartier, 28, of Seattle, a diver who is working as part of the support staff on the project making food for the six men.
She communicates with them through radio - with help from the special recording device, which slows down the divers' helium-altered speech to make them understandable.
The whole operation is neatly packed into a small building built over the opening of a shaft that goes down to the Delaware Aqueduct water tunnel, which was built between 1937 and 1944, and at 85 miles is the world's largest continuous tunnel.
Two crews of three divers work in 12-hour shifts, leaving the chamber through special portals and entering a diving bell that drops them down the shaft.
The men are monitored around the clock through radios attached to their helmets, and closed-circuit TV. A support staff made up of other divers makes their food, cleans their clothes and removes waste from the chamber.
