Just after sundown, a group of curious hikers circles the outer edge of an abandoned mine in the Sodalis Nature Preserve on the edge of downtown Hannibal on a Saturday in October.
Two huge entrances yawn above a trail, blasted into a cliff face. The guide, Gale Rublee, a nature interpreter for Hannibal Parks and Recreation, tells everyone to take a seat and look up: They've reached "bat viewing central."
In the sky, hundreds of bats flit out of two custom metal gates covering the mine openings. The animals silently dance between trees whose leaves are starting to fall and up through the darkening night sky.
In five minutes, the group counts more than 300 bats.
Despite those huge numbers, these flying animals are endangered. They're Indiana bats, and for years, few were known to live in Missouri.
But now, this is the largest overwintering site for Indiana bats in the world. Hundreds of thousands of them migrate to Hannibal each year to mate, fatten up for winter and hibernate.
In fall and spring, visitors can join Rublee's free night hikes to see the swarming bats at the mine entrances. The numbers peak in September and October.
"I know people say Hannibal is known for Mark Twain, but I say if you are in the bat world, Hannibal's on the map," Rublee said.
The discovery
Kirsten Alvey's childhood in 1970s Hannibal was straight out of a Mark Twain novel. She described herself as a tomboy who rode a bike around town exploring local caves, including the abandoned limestone mine on the edge of town.
"I first fell in love with the bats at that quarry in my childhood," she said with a laugh. "I have 'scientific notes' of them as taken by a 9-year-old."
Alvey did an elementary school science project on the bats with help from a librarian. She decided they were little brown bats.
The experience inspired her career. Alvey went on to become a bat biologist and is now the executive director of the Missouri Bat Census, a network of volunteers that works on bat conservation.
In 2012, Alvey was looking for large populations of bats in Missouri to monitor white nose syndrome, a fungal disease that has been decimating bat populations in North America. She decided to go back to that old mine she remembered from her childhood.
The clusters of hanging bats were just where she remembered, but she looked closer and realized she had been wrong all those years ago.
"Oh goodness, these are not just little brown bats," she remembered thinking. "These are actually Indiana bats."
Indiana bats have been federally endangered since 1967. In 2011, the entire known Missouri population of Indiana bats was 13,000. Alvey had found more than that in just one room of the mine.
The bats are small and brown with pink noses. They are known for cuddling up close to each other, and their scientific name is Myotis sodalis, meaning mouse-eared and social.
"This is where people always think I'm weird unless they're a bat person, but I just think they're cute as a bug," Alvey said. "They're just adorable."
Clusters and clusters of bats
Soon after that initial visit, Alvey took Shelly Colatskie, the nongame mammal lead with the Missouri Department of Conservation, to see the mine.
"I just remember seeing cluster after cluster after cluster and I thought, 'Why are these bats endangered?'" Colatskie said. "It was amazing. I've never seen that many in my life."
In the mine, Colatskie found something else surprising — four years earlier, as a grad student, she had put bands on bats in northern Missouri to research them.
"When we went in in 2012, I found my first bat that I ever banded," she said.
Each year, scientists have mapped more of the mine and have gotten better at counting the bat population. More seem to be showing up each year, Colatskie said.
"The Indiana bats are increasing range-wide, so they are doing very well," she said.
At last count, there were almost 260,000 Indiana bats at Sodalis Nature Preserve, which is more than a third of the entire population of the species and is more than double the size of the next largest hibernation site in Indiana.
The scientists aren't sure how the species found the mine, but they say it has the perfect conditions.
"The temperatures in this mine are perfect," Colatskie said. "It's weird. There's enough entrances and the way the wind goes through the site, it just makes it really optimal."
It's also a great location near the bats' migration route.
"Who knows, maybe word has spread in the bat community that Hannibal's the place to come for the winter," said Rublee, the hiking guide. "So the population has grown, which is really, really exciting."
Bat heroes
After the scientists realized how significant the site was for the endangered species, multiple groups sprung into action to protect the land.
The Conservation Fund, a land conservation organization, bought the site with habitat restoration money that had been set aside by an oil company that had built a pipeline in northeast Missouri. The fund then gave the land to the City of Hannibal, and the preserve opened as a 200-acre public park in 2016.
On her nature walks, Rublee tells visitors this is "a hero's story."
"Lo and behold, from 2012 when Kirsten discovered the bats to 2016, we have a 200-acre nature preserve, all thanks to the bats," Rublee said.
The heroes of the story are the bats, the scientists and even visitors to the park, she said. Alvey agreed, saying it has been wonderful to watch Hannibal protect and enjoy the park.
"For me, it's a very big sense of pride to know how much my city has embraced this," Alvey said. "They have really been a good steward to it."
People outside of Hannibal can support bats too, Alvey said, by planting native plants, installing bat boxes and leaving dead trees standing, to serve as homes for the bats.
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