Shimane Prefecture is located in the Chūgoku region of Honshu, the southern end of the main island, along the coast to the northeast. The Oki Islands in the Sea of Japan also belong to the prefecture. Along the coast is a coastal plain but further inland the prefecture is mountainous.
A special feature is the Shimane peninsula which creates two brackish lakes, Lake Shinji and Nakaumi, the second has two islands named Daikon and Ejima. They were created by an eruption 190,000 years ago and have lava tubes. Quite exceptional is, that both islands are actually flat, so the Japanese cannot believe they are volcanic, the Japanese volcanoes are normally high mountains. Since 2017 both islands are a National Geopark and the caves, together with springs, aquifers, the crater of the volcano and several other spots are under protection as geotopes. There are two caves on Daikon island, one is called the 旧洞 (old cave), the other is called the 新洞 (new cave). As they are actually of the same age, the "new" and "old" relate to the time of discovery. 幽鬼洞 (Yuki do, Ghost cave) was discovered in 1926 and 竜渓洞 (Ryukeido) was discovered in 1933. Another version of the names is 大根島溶岩隧道 (Daikonjima Lava Tunnel) and 大根島第二溶岩隧道 (Daikonjima No. 2 Lava Tunnel).