Naruko, Volcanic mountain range in Osaki, Japan.

Naruko is a volcanic group in Ōsaki, Miyagi Prefecture, made up of a roughly 7 km wide caldera with lava domes such as Ogatake (470.3 m), Nakadake (440 m), Kurumigatake (461.4 m), Matsugamine (368 m) and Toyagamori (394 m). Katanuma, a crater lake about 400 m across, lies at its center, and fumaroles remain active around the lake and on the walls of Toyagamori.

Naruko's caldera formed through two pyroclastic eruptions: the 73,000-year-old Nenosaka Tuff and the 45,000-year-old Yanagisawa Tuff. Lava domes such as Kurumigatake and Odake then rose in the center, followed by the shield volcanoes Matsugamine and Toyagamori. The only recorded historical eruption, in 837, was a phreatic blast noted in the Shoku Nihon Kōki.

The name Naruko comes from Japanese words meaning 'a place with a trembling voice', reflecting the volcanic nature of this landscape. Walking through the geothermal areas, visitors experience the connection between the name and the living geology beneath their feet.

The area offers multiple hiking trails suited to different experience levels and physical abilities. Weather conditions and seasons affect which paths are safely accessible, so checking current conditions before visiting helps plan your route.

At the center of the caldera sits a lake with extremely acidic water that reflects the intense geothermal activity still occurring today. The extreme acidity makes it one of Japan's most unusual natural water bodies.