Not every state in the United States has caves.
Several states do not have caves, including North Dakota, Nebraska, Louisiana, Rhode Island, and Delaware.
Most caves in the United States are karst caves.
These caves formed when rainwater, which combines with the carbon dioxide in the air and forms a weak acid, seeped into rock cracks and slowly dissolved the rock, usually limestone.
The water that trickles into the caves contains calcium carbonate, or dissolved limestone.
When that water evaporates, it leaves behind minerals that form stalactites, which hang from a cave’s ceiling, and stalagmites, which build up from a cave’s floor.
In Hawaii, a different type of cave, called a laval tube cave, can form from the lava from a volcanic eruption.
Sometimes, after the lava on the surface cools and hardens, hot lava continues to flow underneath. If this hot lava drains away, it may leave behind a hollow tube that forms a cave.
Sea caves are formed along coastal cliffs when waves containing sand and gravel eventually carve out holes in the rock.