Not as deep as the reaches of Jules Verne's imagination, but proudly touted as the only active undersea laboratory and habitat on the planet, the 85-tonne Aquarius squats in the sand at Conch Reef, 5km offshore and 19m down.
Despite the remote locale, it boasts many of the comforts of home, from air conditioning and a microwave oven to broadband Internet.
The yellow, school bus-sized chamber affords visitors ranging from scientists to NASA astronauts a unique opportunity in inner space - a chance to live on the sea bottom for up to 16 days and scuba dive up to nine hours a day for research on the living reef.
"Aquarius is on the edge, literally. It's expeditionary science that really pushes limits in many ways," said Steven Miller, director of the National Undersea Research Centre at the University of North Carolina at Wilmington. Aquarius is owned by the US National Oceanic and Atmospheric Administration and operated by the university.
Pushing limits is a goal of NASA, which sends astronauts for "extreme environment" training to prepare for space missions; among them is Peggy Whitson, the only person to have spent time on Aquarius and the International Space Station.
"It puts you in an environment that makes you think about the minutiae of living in space"
Peggy Whitson, NASA astronaut
"I was really positively surprised by the number of similarities between Aquarius and the space environment," said Whitson, who led training for astronauts on Aquarius in June.
"It puts you in an environment that makes you think about the minutiae of living in space," she said.
Descent to Aquarius
Arrival at the Aquarius site from Tavernier, Florida, is signalled by a huge floating buoy roughly 9m across that houses power generation and air supply equipment for Aquarius below and wireless telemetry links to shore.
As visitors descend to the depths, Aquarius, sitting on a 120-tonne baseplate, looms amid a cloud of tropical fish. Toothy barracuda loaf in the shadows.
Cables snake through the surrounding sand to nearby experiments, and poles bearing floodlights used to illuminate the depths at night are planted firmly on the bottom.
The lab, 2.7m in diameter and 13m long, shows the scars of the harsh environment where it has been since 1998. Its exterior is mottled with marine growth and scratched by the gnawing of thousands of fish. A maintenance diver chips at the encrustation, an endless task.
Visiting divers enter through the "wet porch," swimming into a passage under the station and poking their heads up into a room filled with air where they take off tanks, masks and fins before showering and entering the tiny living and work space.
"It's got all the comforts of home," said marine scientist Mark Patterson of the College of William & Mary, chatting at the galley table with a visitor to the lab, where he lived for 10 days while conducting research. "It's warm. It's cosy. It's tricked out with e-mail, the Internet, Web cam."
With bunks stacked three to a side and separated by inches, cosy is a polite way of saying cramped. Even Whitson, who lived for six months with two Russian cosmonauts aboard the space station last year, found it confining.
"There's so much to see when you're down here. You ... smell the roses in a way that you just can't when you're swimming by"
Craig Cooper, the Aquarius manager
"We had a lot more room aboard the space station with individual quarters for sleeping," she said.
But Whitson said many of the problems encountered in inner and outer space - communications with Mission Control, "human factors" related to living in tight quarters - were similar.
"Of course there were a whole lot more alien life forms on Aquarius," she said. "The flora and the fauna were amazing."
The key to Aquarius is "bottom time" - the time divers can spend in the increased atmospheric pressure of the sea.
With Aquarius pressurised to 2.5 atmospheres, the same as the surrounding water, scientists can dive and do research for hours each day. At ocean depths of 29 metres nearby, they can spend up to nine hours on the bottom, compared to 30 minutes if they descend from the surface.
At mission's end, aquanauts undergo a 17-hour decompression aboard Aquarius to ready them for a return to the surface.
Smelling the roses
The ability to witness marine life at close range for days on end is an element of the undersea laboratory that cannot be duplicated on shore, scientists say.
Jo Gascoigne, a graduate student who spent time on Aquarius recently, said the snapping of shrimp sounds like rain on the roof of the lab at night. Craig Cooper, the Aquarius manager, recalled spotting a "cleaning station," where fish literally lined up waiting to be cleaned by other fish.
"There's so much to see when you're down here. You have a chance to look around and smell the roses in a way that you just can't when you're swimming by," Cooper said.