Droves of travelers from around the world descend on the Bob Marshall Wilderness Area every year to explore its mountainous terrain, traversing long ridges that pierce the remote backcountry’s most breathtaking scenery.

But for decades a rarefied breed of adventurer has been plumbing the subterranean depths of The Bob’s vast network of karst topography where, nestled into the limestone cliffs of the Chinese Wall, lie some of the nation’s most tremendous caves.

Mike McEachern was among the intrepid few to discover and map them back in the 1970s, hiking tens of miles into the wilderness to explore the sprawling Silvertip Mountain system. Combined, Silvertip and Blood caves twist and turn and fork for more than nine miles beneath the surface.

It took years to link them together, to nose out every entrance and exit and piece together the latticework of caverns, plying every seam, fault and joint with a headlamp beam and descending deeper beneath the scenic Bob, wandering through warrens of dark, narrow passages, among soda straw stalactites and labyrinthine tunnels streaked red with ferrous mineral veins, glittering with diadems of ever-present water seeps and navigating thick layers of clay and mud.

There beneath Turtlehead Mountain lies Virgil the Turtle’s Great House Cave, which, at nearly 1,600 feet deep and more than a mile long, was the second-deepest limestone cave in the continental United States when Jason Ballensky discovered it along with Hans Bodenhamer, a Bigfork High School science and GIS teacher, and two other cavers.

“In the United States, there is a small number of areas that have the potential for undiscovered caves,” Bodenhamer said. “It’s kind of a last frontier in the Bob, and it wouldn’t be that way if it weren’t a protected wilderness.”

At an elevation of 7,130 feet overlooking the White River, Virgil is one of the most challenging caves in the country to access, requiring two days to get there, with cavers carrying 100-pound packs brimming with ropes, helmets, caving suits, and harnesses.

When Ballensky and Bodenhamer discovered it in 2005, a 12-foot wide opening at the base of some dolomite cliffs and a 75-foot long tunnel was the only indication that a cave existed, but then Ballensky uncovered a hidden treasure – a small canyon at the back of the cave that fell off into a borehole, and then another drop into a vast room, which they named the Aurora Room because it was covered in moonmilk, a phosphorescent calcium deposit. (The names of the cave features are as spectacular as the discoveries themselves – there’s Moonray and Sunray and Meanderbelt, Frogg’s Fault and Dover’s Drop and Coffeepot. Lone Goat and Spinshaft, Reaper’s Rock Pile and The Blood Cave.)

Having made the discovery at the tail end of a weeklong exploration trip, the team didn’t have the time or energy to explore and map the cave’s interior.

In 2006, after obsessing over the discovery all winter, the team returned and descended to the bottom and surveyed the length, which at 1,586 feet and more than a mile long, ranked it No. 2 in the continental United States.

Last month, Ballensky bested his own record when he returned to the Bob and descended to the deepest known point of the Tears of the Turtle cave, which is 1,629 feet below the surface, making it the deepest cave in the continental United States.

Since he began caving 15 years ago, Ballensky has been seeking new depth records, confident that the limestone bedding-plane will offer passage to terra incognita.

“The expedition was kind of a culmination of quite a few years working in the area, and the past couple years we had been pushing that cave down pretty hard,” Ballensky said. “We knew the cave could go quite a bit deeper but it was starting to get pretty miserable down there.”

But setting new records and discovering new caving frontier in an underground wilderness wouldn’t be possible if someone wasn’t keeping those records, and for years Bodenhamer and Ballensky have been at the vanguard of using GIS technology to map the caves.

Together, Ballensky, 36, and Bodenhamer, 56, have explored and mapped 94 caves since meeting 14 years ago and, although a pair of decades separates them in age, they share the wide-eyed, childlike fascination with exploring passages no human has ever been before.

“It’s really one of the last places on earth you can really set foot where no one else has been and no one knows what’s there,” Ballensky said. “You can go through some small little room and it opens up into an enormous passageway. It’s wanting to know what is beyond that holds the allure.”

“It’s been a neat underground wilderness saga,” Bodenhamer said.

He’s also turning on another generation to caving, and to that end started the Bigfork High School Cave Club, where students can begin to understand and protect the hidden world.

Many of the students in the club also take one of Bodenhamer’s three courses in geographic information systems and are applying their knowledge of mapping and monitoring to the caving projects. Their subterranean discoveries have led to significant recommendations about how to better manage sensitive cave resources in Glacier National Park and on U.S. Forest Service land.

Knowledge about Montana’s caves has grown expansively due to the Cave Club’s work, and the students’ curiosities are insatiable.

“One of the best things about Hans is how many people he has gotten interested in the sport with his caving club. He has exposed them to this, not only caves but also the geology and all sorts of aspects of science and the outdoors,” Ballensky said.

Caves aren’t a renewable resource, and as a champion of their conservation Bodenhamer has instilled their significance in his students.

“The best thing I think he’s done is expose people to it and get people interested,” Ballensky said. “He’s creating stewards.”