This summer, Richard Brown and five others from around the country hiked almost 30 miles into the Bob Marshall Wilderness to explore an underground wilderness few have seen.

More people have walked on the moon than explored the gloomy, mud-slicked incline of Montana's deepest cave -- Virgil the Turtle's Greathouse Cave. Twelve people have walked on the moon while only nine have been to the bottom of Virgil.

"This cave, without a doubt, is the most challenging one I've ever been in," Brown said. "This is caving on steroids."

The cave's surveyed depth -- 1,586 feet deep and more than a mile long -- ranked it No. 2 in the continental United States. Lava tubes in Hawaii take the top three spots for deepest in all 50 states.

Brown, 58, is the director of product research and development for Wyo-Ben Inc., which manufactures bentonite clay-based products. He started caving sporadically in the 1970s and has been at it hardcore since the mid-1990s. But in all of the caves he's explored, nothing compared with his excursion in July into Virgil the Turtle's Greathouse Cave with the cave's original discoverers and his Billings friend, Ken Stahley.

"The energy outlay doing this cave is very high," he said. "It took us four hours to get down and six to get up, which is opposite of most caves because you're looking around more on the way down."

In the world of cave exploration, finding an unexplored, pristine cave -- especially a large one -- is like a gold miner unearthing the motherlode.

"To find a virgin cave is the holy grail of cavers," Brown said.

Virgil the Turtle's Greathouse Cave was discovered in 2005 by Jason Ballensky, 29, in concert with Hans Bodenhamer, Ben Sainsbury and Philip Rykwalder.

"That was a pretty rewarding experience," Ballensky, who grew up in Miles City, said in a telephone interview from San Diego, where he works as an engineer. "We had spent, at that time, five years looking in the Bob Marshall."

Since the crew found the cave at the tail end of a weeklong exploration, they ran out of time, food and energy to completely map the cave's interior, not even making it all the way to the bottom. So for a year, they were left to contemplate just what they had found, telling few people of their discovery.

"It kept us excited all winter," Ballensky said.

What they had discovered was a huge room more than 100 feet wide in places and with the roof in spots rising 30 to 100 feet above. In an article about the discovery in the February issue of the National Speleological Society News, Rykwalder wrote of the room, called a borehole. "A borehole as big as Virgil's is not common anywhere, period."

Returning in the summer of 2006 with a larger group, the cavers re-entered Virgil and diligently mapped and photographed its extensive interior, the dimensions of which they wouldn't fully know until adding up the numbers after returning to civilization. The result: Virgil is the second-deepest limestone cave in the United States and only 18 feet shallower than the deepest one, Lechuguilla Cave in New Mexico.

But according to Ballensky, Virgil is by far the most challenging cave in the country just to get to.

The entrance to Virgil sits at an elevation of 7,130 feet overlooking the White River. It takes two days just to hike there.

"The whole area up there is unique," Brown said. "There's a lot of limestone, and anywhere you have limestone there are caves. But not all of them have entrances so you can see them."

The cave is located at the base of dolomite cliffs, its 12-foot-wide opening running about 75 feet back.

But its hidden treasures, deep below, weren't so easily revealed. Rykwalder wrote, "It was as if the cave was a Machiavellian sorcerer who had woven protective spells and made hazardous obstacles to ward against entrants, regardless of their intent."

It was Ballensky who discovered a small canyon at the back of the cave after climbing 6 feet. A few hundred feet later, the canyon fell off into a borehole that gave Ballensky, Rykwalder and Bodenhamer their first clue that they had found something unusual. After another drop, which required setting an anchor and rapelling, the discoverers found another large chamber, this one covered in moonmilk, a phosphorescent calcium carbonate deposit. It was christened The Aurora Room.

"I've dreamed of deep caves," Rykwalder wrote, "but never in my wildest fantasies have I envisioned anything this grand."

"You're usually hoping to find something that's never been seen before," Ballensky said. "In this case, it was how the cave developed."

Exploration proved difficult. Bus-sized boulders littered the cave floor.

"It's like bouldering except with a roof over your head," Brown said.

The temperature was a constant, hypothermia-inviting 38 degrees. Drops were 15 to 70 feet, the largest of which is named the Death is All Around You Drop. And tight crawls were so confining and in danger of caving in that one was named the Sarcophogus Crawl. Descending the passage requires walking at a constant pitch of 30 to 60 degrees on shed rock that makes footing difficult.

"The loose rocks and mud at the bottom of the cave is a little bit treacherous," Ballensky said. "But it definitely could be worse. There are other deep caves in the U.S. that have taken years to explore. We did this in a couple of days. There were very few places where we needed to use ropes. That saved us a considerable amount of effort."

Ballensky said that as the explorers continued down, they kept thinking they had found the end of the cave, only to discover a means to continue on.

Almost three-quarters of the way into the cave, a cold stream of water was named the Nutzowhyt River. Its waters carved through the clay cave floor, exposing a sedimentary layer more than 5 feet tall.

"It has striations like rings on a tree," Brown said. "Each has to indicate a significant event."

But all of that didn't compare to what the cavers found in the bottom quarter of the cave -- mud 4-6 inches deep, in some places so soft that the explorers would sink up to their knees.

"You'd grab on with your heels and fingers to keep from falling into the pits," Brown said.

The goo clung to the cavers' suits, weighing them down with pounds of mud and amplifying their exhaustion.

"If someone had gotten hurt, it would have been more than a major evacuation," Brown said. "You're extra cautious. You're aware of your surroundings a bit more than you normally are, which is a lot."

It was at this point that Brown wondered, "Do I have the energy to get back up?"

The cave ends somewhat unceremoniously in a sump, a small outlet for water that enters the cave.

"It's obviously glacier created," Brown said of the cave. "It's impossible to tell how old the cave actually is. Certainly, the last glacial period, from 10,000 to 1.5 million years ago, had a lot to do with its formation. We think that the clay deposited in the bottom half of the cave may have resulted from sediment that was carried into the cave by glacial melt water. Each band in the clay layer may represent a separate flooding and deposition event. There are a lot of them so this obviously went on for a very long time.

"There are a lot of other caves up there," he added. "So far, none of the others are as extensive, but there's always a hope that another one will be found."

Ballensky and friends plan to return this summer.

"We're pretty confident there are more out there," Ballensky said. "There are a couple more pretty decent-sized caves still to be found."

Brett French can be reached at french@billingsgazette.com or at 657-1387.