Thousands of hibernating bats have survived Minnesota winters by holing up in one of St. Cloud's oldest stormwater tunnels. Built in the 1920s, the almost ornate brick-and-mortar sewer opens to a ravine that pours into the Mississippi River.

But the tunnel has failed to keep up with the growth of the city. With all the new homes, roads, driveways and other paved places added over the last century, too much water is now being sent through it, drowning out any creatures that may be sheltering inside and eroding the ravine where it empties.

All the bats that once roosted inside have moved out.

Now, as more than 90% of North America's hibernating bats have been killed off by a plague-like fungus, the city hopes to turn the old sewer back into a functional bat cave. To do that, the city would build a new storm sewer, relieving pressure on the old tunnel, which also has an erosion problem that threatens properties downstream. It could offer some respite for the few surviving bats in Minnesota, said Lisa Vollbrecht, St. Cloud public utilities director.

"It's a neat opportunity to restore a hibernaculum where they have not found the fungus," Vollbrecht said. "It's not an active population anymore, but back in its day it was thriving."

The first hibernating bat was found in the St. Cloud sewers in 1952. The Minnesota Department of Natural Resources (DNR) and St. Cloud State University conducted periodic surveys of the hibernating colonies over the years. But by 2016, DNR biologists found that the tunnel was flooding to its top in the spring, making it unsuitable for roosting bats.

That's around the same time that the fungus, which causes the devastating white nose syndrome, was beginning to take hold in the rest of the state.

The fungus spores, which originated in Europe, attach themselves to clothes, skin, the bottoms of boots and shoes, and eventually find their way to a bat's fur. The animals will live with it, unbothered, all summer while they are active and eating regularly. It typically spreads around entire populations.

Once they hibernate, their immune systems shut down to save energy. The fungus starts to spread into little white spots on their noses, or the tips of the their wings. The bats wake up to scratch the sore spots or to try to lick it clean. The amount of energy they lose by waking up is enough to starve or dehydrate them. They either die in the cave after failing to get back to sleep, or fly out in a last, desperate search for food in the dead of winter.

A study this month from the U.S. Fish and Wildlife Service found that North America has lost more than 90% of three of its most common hibernating bats species in just the last 10 years.

The DNR's annual estimates on Minnesota populations are just as bleak, losing about 98% of the bats in some of the largest known hibernation spots, including in the Soudan Underground Mine near Lake Vermilion.

But there are signs of hope, said Ed Quinn, natural resource program supervisor for the DNR's parks and trails division. In some eastern states, which have been fighting the fungus for a few years longer than Minnesota, populations are starting to rebound.

"Out east, they're finding bats that have had symptoms, survived and are reproducing," Quinn said.

Some biologists believe the bats of Europe may have undergone a similar plague, with nearly all dying off, when the fungus first arrived there, Quinn said. But those few that survived were able to breed more bats strong enough to fend off the fungus. Bats in the eastern United States may just be starting that recovery, he said.

While there may only be 2% of the bats left in much of Minnesota, the animals are still hanging on.

"We're hoping that we're at the bottom, and we're going to start to see some rebound," Quinn said. "We've got a small number of survivors, so we need to do the things we can to help maximize their reproductive effort."

In St. Cloud, that could mean preparing an ideal habitat for their comeback in the old sewer.

Rerouting water around the St. Cloud sewer would take about a year to complete. The city estimates the cost would be about $3 million. It is seeking $1 million from the state's Environmental and Natural Resources Trust Fund to restore the bat cave and $2 million from the Federal Emergency Management Agency to protect structures from the eroding ravine.