Sea caves are produced when a number of reentrants join behind the face of a cliff, leaving behind supporting pillars and arches. They develop most easily where the sand layers comprising a rock formation are very thin. The thinly bedded, easily eroded sandstones of the Devils Island Formation are the source of Apostle Islands National Lakeshore's spectacular array of sea caves. The caves' beauty varies dramatically with the season. In summer the red sandstone is sandwiched between sapphire blue lake and emerald green forests that grow right up to the brink of the cliffs.
Large waves generate plumes of spray and thunderous explosions as they surge into the sea caves.
While visitors must enjoy these scenes from a distance, such is not the case when the lake is tranquil. Under calm conditions, kayakers can explore the caves' deepest recesses while listening to the murmer of water against rock. By February, an ice bridge might connect Sand Island to the mainland. The lake surface is usually a frozen white expanse. Lakeshore cliffs form a crimson red border to this arctic landscape.