FORT STANTON CAVE, N.M. (AP) - Hundreds of feet beneath Earth's surface, a few seasoned cave explorers venture where no human has set foot. Their headlamps illuminate mud-covered walls, gypsum crystals and mineral deposits.

The real attraction, though, is under their shoes.

A massive formation that resembles a white river spans the cave's floor. A closer examination reveals that the odd formation is an intricate crust of tiny calcite crystals.

The explorers have reached Snowy River - thought to be the longest continuous cave formation in the world.

"I think Snowy River is one of the primo places underground in the world and there's still so much left that we haven't discovered. ... We don't even know how big it is," said Jim Goodbar, a cave specialist with the federal Bureau of Land Management.

The survey expedition by members of the Fort Stanton Cave Study Project in early July added several thousand feet to the measurement of the spectacular formation, which is at least four miles long. The explorers who have been following the passage under the rolling hills of southeastern New Mexico say there's still more of Snowy River to be discovered.

The few who have walked on the formation say they've seen nothing else like it. Early studies point to its uniqueness: Already, some three dozen species of microbes previously unknown to science have been uncovered.

New Mexico's two U.S. senators are pushing for Congress to designate Fort Stanton Cave and Snowy River as a national conservation area. The designation would protect the area from such activities as mining that threaten the water flows that created the cave. It also might generate funding for scientific research.

"It's certainly a national treasure and very well worth protecting in its own right, even without Snowy River. With Snowy River, it puts it in the class of world-class caves," said John McLean, a retired hydrologist and member of the cave study group.

"It's a beautiful anomaly," added Penny Boston, a New Mexico Tech professor and associate director of the National Cave and Karst Research Institute.

Boston says extreme environments such as Snowy River provide scientists an opportunity to explore life on the fringes.

"The idea is that we're practicing to go to Mars, we're practicing to go to Europa (a moon of Jupiter) and all of these other places," she said. "It's very difficult to even prove some of the things we've studied here on this planet are alive. Imagine how much harder that is when you translate that to a robotic mission millions of miles from Earth."

Boston has collected microorganisms that she believes are responsible for the manganese crust that covers much of the walls in the Snowy River passage. Once thought to be ancient and inactive, the microbes are busy in her lab, breaking down materials and producing mineral compounds.

Boston and other scientists plan to take core samples of Snowy River to look for microbes that have been entombed in the calcite layer and for fossil evidence of past microscopic life.

Some scientists are looking to the cave to learn more about the region's geology and how water makes its way through the arid environment.

Last summer, explorers were surprised to arrive at Snowy River and find it flowing with water. It had been dry when first discovered in 2001 and during trips in 2003 and 2005.

It took several months for Snowy River to dry out, leaving scientists with another set of questions about where the water came from and where it went. Some scientists believe innumerable floods formed Snowy River, dropping a thin layer of calcite each time.

Areas of Fort Stanton Cave are open to those who get permits from the BLM, but Snowy River - deep in the cave behind locked metal gates - is off-limits. It's unlikely Snowy River ever will be open to anything but research because of the fragility of the tiny calcite crystals and microbes on the cave walls.

On the Net:

Bureau of Land Management: http://www.blm.gov

Fort Stanton Recreation Area: http://tinyurl.com/5onjnc

National Cave and Karst Research Institute: http://www.nature.nps.gov/nckri/