Mammoth Cave is known as the worlds longest known cave system with more than 400 miles of surveyed passageways. Many of you have probably been to Mammoth Cave, as the park typically sees over half a million visitors a year. 

This past weekend however, some visitors got to see a rare and interesting sight within the cave itself. Park officials stated that the popular Rotunda room in the cave was filled with fog on Sunday after a storm moved through the park.

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This special site isn't one that occurs very often within a cave as the temperature inside caves typically doesn't drop dramatically and help fog to form. However, that happened on Sunday. 

The park said in a tweet "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!"

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The Rotunda room that the fog formed in is one of the most visited places within the cave. The room is the sixth largest in the cave and total is about a quarter of an acre in size with it being about 40 feet from floor to ceiling and around 140 feet wide.Â