The city of Decorah, Iowa is home to a rather amusing and unusual geological wonder. The Decorah Ice Cave is located in a small park in a bluff on the north side of the Upper Iowa River. The bluff is composed of thick limestone. Water is able to percolate through fissures in the limestone. Some of that water finds its way into the Ice Cave.
During the winter months, cold air fills the cave. This cold air becomes trapped at the bottom of the cave. As winter gives way to spring, the cold air remains in the cave. Even during the summer months, the cave will be considerably colder than the outside air.
Remember all of that rainwater percolating through the limestone? Due to the cold air in the cave, it freezes on the walls and the floor of the cave. Thus, much of the cave becomes coated with ice in the winter. At least some ice will often last into the summer months.
The cave is open to the public, but you should use extreme caution if you hike into it. You will want to bring a flashlight, as the cave is unlit. In addition, the ice can make the floor of the cave very slippery. Be very careful if you visit in the winter or spring. In addition, children should be closely supervised at all times.