WESTERN PENNSYLVANIA—A few hours outside of Pittsburgh, Pennsylvania, there’s an abandoned, partially flooded railroad tunnel with a reputation for being haunted. Decades’ worth of graffiti warn the curious to stay away, while inside, a long corridor of fetid water sloshes about my knees as the daylight disappears.

And it’s true what they say about the place, by the way. An invisible killer lurks within.

Scientists call it Pseudogymnoascus destructans, or Pd. But it’s also known as the flesh-eating fungus that’s been annihilating bat populations across North America. In just over a decade, it has killed off upwards of six million bats.

But scientists aren’t giving up hope yet. In fact, there’s a slate of clever new projects in the works that are testing