Crater Lake, Oregon, is in a crater-lake caldera, not a crater. The caldera is around 6 miles across. Crater Lake is the deepest lake in the United States. It was formed by an eruption more than 7,000 years ago at Mount Mazama. The eruption was so large and violent that it emptied the volcano’s magma chamber. The magma chamber’s roof collapsed and eventually filled with water, creating the lake. Smaller, later eruptions built up Wizard Island, a volcanic cone that forms an island in the lake.
Deception Island, located off the coast of Antarctica, is another crater-lake caldera. The Deception volcano is still active. It violently erupted about 10,000 years ago forming a caldera about 4.4 miles across. One wall of the caldera was breached, and the caldera then flooded with seawater, giving the island its horseshoe shape. Deception Island’s unique geologic structure makes it one of the only places in the world where ships can sail directly into an active volcano.