ELKMONT, Tenn. — Lynn Faust remembers the old days of firefly season here.

You would hike into the woods at night, with nobody else around, waiting for one of nature’s strangest and most beautiful rituals. Then the fireflies would emerge, thousands and thousands of them, and under the moonlight they would all flash in unison. On. Off. On. Off.

“It’s as though they wear little watches,” said Ms. Faust, 56, a biologist and naturalist who has studied fireflies for decades. “It’s awe-inspiring, it’s beautiful, it’s rhythmic and it’s bright. You’re surrounded by the fireflies.”

These days, you are also surrounded by the tourists.

The secret is out about this marvelously rare and very brief annual spectacle. About a thousand tourists a night come to Elkmont, a small trailhead in Great Smoky Mountains National Park, during the two weeks each June when the country’s largest population of synchronous fireflies puts on what locals call “the light show.”