Most caves are formed in limestone or other soft or soluble rock by water that trickles through cracks and dissolves or wears away the stone. Some caves are made by underground streams that flow through openings in the rock and make them larger. Lava caves form in the sides of volcanoes when lava on the surface becomes cool and hard while hot lava still flows underneath.
Ice caves form in glaciers and icebergs when ice melts and freezes.
Mammoth Cave in Kentucky is one of the best known caverns in the United States. Other noted caves of the United States include Luray Caverns in Virginia, Wyandotte Cave in Indiana, and Wind Cave in South Dakota. One of the largest is Floyd Collins Crystal Cave in Kentucky, with 32 miles of passages.