A Maryland engineer plans to lead an expedition next year deep into the core of a 6,500-foot-high limestone plateau in Mexico, wiggling through crevasses, climbing down thundering waterfalls and scuba diving through uncharted caverns.
The payoff? William Stone and the 11 other expert cave climbers and divers on his international team hope to travel more than a mile beneath the surface, deeper into the planet's crust than anyone else has ever gone.
"This ranks with [Sir Edmund] Hillary's climbing of Mount Everest," said Heather Sloan, a spokesman for the U.S. Deep Diving Team, one of the expedition's sponsors.
Mr. Stone, 37, who develops earthquake-resistant bridge and building designs for the National Institute of Standards and Technology in Gaithersburg, has been exploring the Huautla Plateau's cave system since he was a graduate student at the University of Texas in the mid-1970s. The plateau, pronounced "Wowt-la," is about 170 miles southeast of Mexico City.
"Basically, we were the first explorers of Huautla," said Mr. Stone of Derwood in Montgomery County. "And we've followed it through for the last 15 years. Now, the extent of what is known has been pushed pretty hard. We're dealing with the very limits of life-support technology."
Other cavers have spent time mapping Huautla's serpentine open-air caverns, their walls as smooth as glass, which were carved over thousands of years by water cascading from the surface or boiling up from below.
"It's like walking through a canyon full of rock that's polished like a lab specimen," Mr. Stone said. "It's very dark and foreboding."
But friends say the explorer has obsessively sought the lowest reaches of what he thinks could be the world's deepest cave system.
A deep underground river made further exploration impossible without more sophisticated scuba technology, Ms. Sloan said. So Mr. Stone taught himself life-support engineering and began designing his own computer-controlled breath-recycling backpacks.
Those "rebreathing" units -- which include gas cylinders, six tiny computers and a miniature gas-processing plant -- can sustain a diver for more than 12 hours. Ms. Sloan said they are designed to be safer and last longer than rebreathers used by astronauts or SEALs, the Navy's special forces.
"He's very bright, he knows his field, and he's a real innovator in the way of dive gear," said optical engineer John Schweyen of Glen Rock, N.J., an experienced cave diver and a member of the 1992 expedition.
The explorers say they want to map the cave because they like the challenge, the thrill of discovery and the chance to break the world record for deepest cave exploration, which Mr. Stone said is 5,253 feet.
They also hope to use surveying equipment to precisely locate the underground river so that an Indian tribe on the surface can gain a reliable water supply with deep wells.
For seven years, a nameless underground river that drains the plateau has stymied efforts by Mr. Stone and his colleagues to reach deeper into the cave's main section.
Flowing through flooded caverns, the river reaches another cave system, the Cueva de la Pena Colorada, and eventually emerges in a rain forest in Santo Domingo Canyon, eight miles away.
Cavers descending from the top of Huautla reach the river by swimming through a connecting pool, called the San Agustin sump, that sits above it.
The deepest Mr. Stone has reached from the top of the plateau is 4,439 feet, to a point the equivalent of about nine stories beneath the surface of the sump in a flooded cavern about as wide and deep as a subway tunnel.
That, Ms. Sloan said, is the point at which "every person on this expedition knows that, should anything go wrong, there is almost no hope of rescue."
In late February, the Huautla team -- which includes three British, two Mexican and seven American cave experts -- hopes to begin a 2 1/2 - to-three-month expedition from a pine forest at the top of the plateau.
Cut off from radio contact with the surface, they will haul tons of equipment through the cave, inching down the sides of 10- and 20-story waterfalls and diving through other sumps that block the passage.
While climbing down the waterfalls, Mr. Stone said, cavers must use whistles to communicate over the roar of water bouncing off the marble walls. To ward off fungi, they will have to wash their clothes in an iodine solution. To kill bacteria on their skin, they will bask in lightweight ultraviolet lights.
At the San Agustin sump, which is surrounded by sheer rock walls, they plan to create a base camp by suspending a web of ropes a few feet above the pool's surface. There, they will sleep and eat in hammocks while preparing to search along the subterranean river for more caverns.
The explorers hope that will be long enough to take them from the sump to a network of dry caverns where they will set up another camp and push out to survey more of the cave.