Thousands of bats return to this dark refuge each fall to hibernate and wait out the winter. Mystery Cave has sheltered them for centuries.
This fall, though, something in the cave is threatening to kill them all.
A fungus that eats at the bats' wings has been found here and in Soudan Underground Mine, two of Minnesota's largest bat enclaves. In East Coast states, the white nose fungus has wiped out nearly 6 million bats.
State biologists still hope the bats here might survive. They're looking for ways to contain or beat back the fungus. If not, the furry, peaceful creatures resting on the cave walls will likely die in the next few years.
If that happens, there will be losers beyond the bats, Mystery Cave Park Manager Mark White said as he led a visitor recently through the caverns.
"You're just going to see an explosion of corn-borer moths, mosquitoes, midges, other night-flying insects, some aquatic insects," White said. "So it'd be a shame to lose them."
Mystery Cave is tucked like a Hobbit home into a steep hillside. Oaks and maples glow in fall colors stand on the ridge. Above the doors to the cave, an opening like a big mail slot invites bats to fly in and out.
This cave is 48 degrees year-round, a few degrees warmer than a refrigerator, with a constant supply of fresh air and occasional drips of water on the head. Shoulder-width passages open into spacious rooms with ceilings 20 feet high.
Some walls are pock-marked with holes left by fossilized creatures when this limestone formed the sea-bottom. Others have the texture of stucco -- cavers call it popcorn. The edges of thin shelves jutting from the walls are decorated with glinting crystals, like a frilly skirt.
White plays his flashlight on the walls and ceilings. The beam occasionally picks up tiny, brown, soft-looking, mouse-like creatures hanging from the walls. White says he often marvels at how the bats can hang here, sleeping, for months through the winter.
"They just grab little imperfections in the rock, where there's some texture in the rock and hang onto it with their little nails," he said. "They don't ever seem to fall off, or it's very rare to find one that's fallen off."
White shines the flashlight just to the side of each bat. He says if the beam falls directly on the bat, it will warm the creature enough to disturb it.
About 2,000 of Minnesota's most common bats -- little brown myotis bats -- make their winter homes here. So do a handful of the less common big brown bats and tri-colored bats, also known as eastern pipistrelles.
They live all winter on the fat stored from copious meals of night-flying insects. They spend their summers outside the cave and at this time of year they go in and out.
Since 2006, when the fungus was first discovered in the U.S. in a cave in New York, 6 million bats from New York to Georgia to Ontario have been effectively wiped out.
By one estimate, their insect eating amounts to a billion dollars of free pesticide control for local farmers.
"As long as it's warm out they'll head back out to eat insects," said White. "Then at the point where it gets cold enough there's no bugs out, they're smart enough to know, 'I'd better fly deeper into the cave and do my permanent hibernating for the winter.'"
Last winter, when biologists counted the hibernating bats, they swabbed the animals' fur. In August, bad news came back from the lab: One or two out of 20 sampled bats were carrying the fungus Pseudogymnoascus destructans -- also called Geomyces -- that causes white-nose syndrome, named for the discoloration in creates on the bats.
The fungus is native to Europe, and scientists think human visitors brought it to American caves. As with other foreign invaders, like gypsy moths and emerald ash borers, researchers are scrambling to find a way to fight back.
Since this fungus was first discovered in a cave in New York in 2006 it has effectively wiped out nearly 6 million bats from New York to Georgia to Ontario.
The most common little brown bats are thought to be the most sensitive to the fungus.
Even if a few survive, they might not be able to maintain a viable population because they only have one baby each year. And there aren't enough of the other bats to take care of the job of killing destructive insects.
There's some hope in Minnesota because only small amounts of the fungus have been found so far, and because Minnesota's bats don't congregate as closely as bats in eastern caves.
Researchers are delving now into the genetics of the fungus, the physiology of the disease, and possible conservation and recovery efforts.
At Soudan Underground Mine State Park in northeastern Minnesota, University of Minnesota' researcher Christine Soloman is working with microbial organisms in the mine she thinks might inhibit growth of the fungus.
In southeast Minnesota, Mystery Cave's managers plan to install decontamination equipment -- essentially a strip of carpet to dislodge spores from shoes, and a big sponge filled with soap or spearmint to kill the spores -- for visitors to walk through as they enter and leave the caves.
Researchers will sample bats this winter to see if the fungus is growing.
Hopefully, scientists can clarify how the disease progresses and possibly find ways to stop it, said Gerda Nordquist, a Department of Natural Resources biologist.
"You'd better hope," she added. "We do not want to lose entire populations of a native animal to some non-native invasive organism. We've got to find an answer to this."