The limestone (dolomite) at Cave of the Mounds has formed 400-500 million years ago when the area was covered by a warm, shallow sea. This “Ordovician Ocean” was teeming with life: cephalopods, brachiopods, crinoids, and trilobites. As these creatures died, their remains would sink to the bottom of the sea. The leftover bits of animals, plants, and sediments were compacted and cemented together to form limestone. The caves in Southwestern Wisconsin were formed by flowing water that seeped through the natural cracks in the limestone. It has taken huge amounts of water and a long time to form Cave of the Mounds!