Mount Katmai, Stratovolcano in Kodiak Island Borough, Alaska.

Mount Katmai is a stratovolcano on the Alaska Peninsula with a broad caldera that now holds a deep lake. The crater walls rise steeply and show layers of ash, lava, and volcanic rock, while the lake rests quietly in the center.

The major eruption of June 1912 shaped the caldera and blanketed the surrounding area with deep ash. In the years that followed, the crater filled with rainwater and snowmelt to form the lake seen today.

The name Katmai comes from the Alutiiq language. The caldera walls show tan and rust-colored patches on the upper faces above black rock at the lower edges, and the lake shifts from turquoise to pale milky green along its right side. The 1912 eruption was first attributed to Katmai, though its vent lay at Novarupta, 10 km west.

Access is by floatplane or boat, as no roads reach this part of the national park. Visitors should be prepared for rapidly changing weather and bring warm, waterproof clothing for all seasons.

The crater lake hides an island of lava rock that only becomes visible when water levels drop. Geologists discovered it years after the eruption, when they began mapping the lake floor.