Dogashima area offers unique chances to observe traces of subaqueous volcanic debris flows from several million years ago. In those ancient times, Izu Peninsula was a collection of submarine volcanoes, causing frequent debris flows on the seabed. You can tell that debris flows took place under the sea, due to the fact that bigger rocks sink in at a lower level of the flows. Dogashima area demonstrates many good examples of such debris flow activities. Tensodo Grotto, in particular, boasts its mystical beauty carved out and exposed by ocean waves.