Suddenly one night each spring, swarms of fireflies in the Carolinas start flashing in unison, streaming light in waves.
It could be tonight.
The marvel is a seldom-seen synchronous display, a mating ritual of only three species among literally thousands of types of fireflies. Two of them live here.
The more famous one draws so many people to the Great Smoky Mountains each spring that the National Park Service hands out parking spots through a lottery.
The other is starting to draw so many people to the Congaree National Park near Columbia that staff there is looking at a shuttle system.
But here's a little-known secret: The fireflies in the Congaree also are found in the Lowcountry. They might be lighting up at the edge of a nearby woods right now — researchers just don't know enough about the species to be sure.
The chances of you catching sight of the display aren't good. But you might get lucky.
One thing the people who have seen them do know, "it's like seeing Christmas lights in the middle of the spring. It's magical," said Ranger Jon Manchester with the Congaree park.
Congaree expected the fireflies to start gleaming sometime around May 10 and peak a week or two later. The display could continue through Memorial Day, Manchester said.
The Great Smoky Mountains National Park is calling for a peak right at the end of the month. The Elkmont area of the park near Gatlinburg, Tenn., is considered ground zero. Access is closed off at night during the display, except for the 1,800 lucky parking lottery winners who already have been chosen.
Almost 30,000 people applied for those permits this year, Manchester said.
Overall, fireflies are considered to be in a long decline from factors such as habitat loss, pesticide use and even lawnmowers. The biggest thing needed for synchronous flashing to get going is a lot of fireflies, said JC Chong, a Clemson University entomologist who has studied the insects.
"Although in our eyes the fireflies are flashing synchronously, the individual firefly is just doing what he or she has always done," Chong said.
"As one individual is flashing, we don’t perceive that as synchronous flashing. But if a large number of fireflies are flashing, even within a few milliseconds of each other, in our eyes we are seeing synchronous flashing," he said. "We just do not (usually) have enough fireflies congregating at the same location at the same time to perform the feat."
But they are out there.
"We have a fairly good indications the displays have been seen in other places along the Congaree River," Manchester said.
They also have been seen in other spots in North Carolina, in Tennessee and in Pennsylvania. Enthusiasts advise people to look for the light show in cove forests near small streams, avoiding ridge tops and meadows.
The fireflies like woods and don't like competing light, so go somewhere dark in the trees and keep it dark.