A great way to explore the Dogashima area in Izu is to try the cave excursion cruise operated by Dogashima Marine. This cruise will take you to the Tensodo cave, also known as the blue cave, a designated historical monument. The cave is 147m long and is illuminated by sunlight entering the cave through a hole in the ceiling, which makes for great photographs. Note that the cruises may be cancelled in case of bad weather.