Together with Satsuma, Osumi forms the most southerly part of Japan, excluding the Ryukyu Islands. It has many active volcanoes, with Sakurajima being the most unstable. The volcanic soil and frequent ash deposits actually make the place rather fertile. The island of Tanegashima, where the Portugese captain Mendes Pinto was stranded in 1543, belongs to the Osumi Islands. For a long time all muskets were called "tanegashima", as gunpowder in Japan was thought to originate from here.