MAMMOTH CAVE, Ky. (WKRN) — A group of visitors at Mammoth Cave National Park in Kentucky got to see a rare sight on Sunday when a thick cloud of fog formed underground in the Rotunda at Mammoth Cave.

Molly Schroer, the Public Information Officer at Mammoth Cave National Park, says this rare phenomenon only happens when conditions are exactly right, and it doesn’t last long.

“It was only in there a short time, about 10 minutes. And then the cold cave air came in and dissipated it and it went away,” she said.

Caves have their own natural airflows, and in the summer you usually have cool cave air pushing out of the entrance. But on Sunday, that airflow reversed.

“It was a nice warm summer day; a storm came through that dropped the temperature, lowered the pressure, and that cave, instead of pushing air out actually sucked it into the cave and stuff(ed) that warm moist air and the fog in and it collected in the first big room that you can see in the picture called the Rotunda, and gathered up near the ceiling,” Schroer said.

This isn’t the only interesting weather that happens in and around the cave.

“Different times of the year at the mouth of the cave, fog will gather at the entrance [which is where] we’re doing tours in the early morning. And that’s a really neat experience to feel the air as you go in, and it’s nice and cold at your feet, and in the wintertime when it’s cold outside and icicles everywhere,” said Schroer.

Tours of Mammoth Cave are offered daily during the summer months.