Caldera occupied by Lake Tazawa, seen from Mt. Komagatake, Backbone Range. -- Lake Tazawa, the deepest lake in Japan, is nearly circular in shape and about three and one-half miles across. It is nearly 1,400 feet deep and occupies a large depression of volcanic origin, called a caldera. This depression has steeply sloping sides and a mostly flat bottom. The caldera formed when huge amounts of volcanic material were erupted leading to the collapse of the resulting volcano into the vacated, empty chamber below. At least one small cone, formed by later volcanic activity, rises above the lake floor, but not to the lake surface. There are other calderas in Japan, including Lake Towada north of Morioka. Crater Lake in Oregon is a similar feature in the United States.