Every June, more than 10,000 people descend upon Great Smoky Mountains National Park’s Elkmont Campground to see a miracle of nature: a rare species of synchronous fireflies that flash in unison. Visitors stand in line for hours to be shuttled in and out of the campground, which sits in a river valley on the western edge of the park. The Elkmont fireflies are the most well documented and stunning example of synchronic bioluminescence in the U.S., but the Smokies doesn’t have a monopoly on in-sync bugs. Another population of synchronous fireflies was recently discovered in Pennsylvania’s Allegheny National Forest.

The fireflies found in both the Smokies and Allegheny flash several times in unison, go dark for several seconds, then flash again in harmony. “They look the same, act the same, have the same DNA,” says Lynn Faust, leader of the team that studied the Pennsylvania lightning bugs in 2012.

Faust, a Tennessee native, was responsible for bringing the Smokies’ population of fireflies to the attention of the scientific community in the 1990s. She has spent the last 22 years traveling the world to study fireflies of all kinds. “It’s a hobby that went nuts,” she says. Faust wasn’t surprised to see the fireflies in Pennsylvania and says, historically, the lightning bugs have been seen throughout the Southern Appalachians.

“I have scientific papers that date back to the 1850s that mention reports of the synchronicity. I’m surprised we don’t have more reports from other parts of Appalachia.”

She’s personally seen them in several locations outside of the Smokies. Just don’t expect any other show to be quite as spectacular as what’s found in Elkmont.

“The show in Allegheny is impressive, but the fireflies are more spread out,” Faust says. “In Elkmont, thousands of them can be found in a condensed area. I’ve seen fireflies all over the world, but you can’t beat the show at Elkmont on a peak night.”

See For Yourself 

The synchronous fireflies typically peak in mid-to-late June, depending on the weather patterns leading up to their mating season. In Elkmont, you’ll have to register for a shuttle to see the lightning bugs. In Allegheny National Forest, the Allegheny Defense Project is organizing a Firefly Festival this year to celebrate the bugs on June 21-23. Find out more at alleghenydefense.org

Or head into the woods to discover a colony of synchronous fireflies of your own. Synchronous fireflies only live above 1,600 feet in elevation and prefer elevations between 2,200 and 2,400 feet. They are typically found near rivers in older forests without a lot of understory, so they can see each other.