Being underground doesn't make you immune to the weather.
Park officials with Mammoth Cave National Park shared a cool photo on Sunday that shows fog in the Rotunda Room.
"Today we had a large and unexpected storm come through the park which caused a quick drop in the temperature. This pressure change caused the airflow to reverse, pulling into the cave, resulting in a dense fog filling the Rotunda room!" the park shared on Facebook.
The Rotunda Room is one of the most popular part of the cave and is 180 feet below ground.
Cave visitors have to be very lucky to see this rare sight. Park officials say it only happens when certain conditions are met in the cave and on the surface.
The fog lasted for about 10 minutes before it disappeared.
One person on Facebook pointed out that it doesn't look like fog, but "That is clearly a portal to another world."