Tatami-ishi 畳石 (Tatami-Rocks) is a Nationally Designated Natural Monument located on the south coast of Oujima 奥武島 (Ou Island). A bridge connects Oujima with Kumejima. Andesite Magma cooled under a process called columnar jointing creating 1 to 1.5 meter pentagonal and hexagonal shapes. Called Tatami-Rocks because of the resemblance to the tatami mats of traditional Japanese home design. Water erosion has leveled and smoothed the tops of the rock structures.