Crater Lake, Oregon, USA
U.S. Geological Survey photo (fair use policy)
Crater Lake fills the caldera of prehistoric Mount Mazama, one of the Cascade Range volcanoes. Calderas mark a rather temporary stage in the life of a volcano that happens when the body of magma underground empties out, either because it erupted or because it drained for some reason. The volcano itself, being a weak structure, falls into the vacancy. In this case, the caldera formed a very nice bowl, with no openings on any side. Most calderas aren't so tidy (see another example here). That allowed it to fill with 600 meters of water, making it the deepest lake in North America. But in a few thousand years or so, eruptions will begin again and blow out the water, or one of the walls will erode to drain the lake, and Crater Lake National Park will have a silly-looking name.
On the far side of the lake is a small cinder cone named Wizard Island. Also breaking the water surface is a small, ragged dike fragment, the Phantom Ship, which has its own photo spread.