Lake Nakatsuna, Tectonic lake in Ōmachi, Japan

Lake Nakatsuna is a tectonic lake in Ōmachi, Japan, formed by movements of the Earth's crust. It sits in a valley near the Northern Alps, with clear water and a shoreline that shifts from reeds to open water.

The lake formed over a long period as the land shifted, and it has been part of the local landscape since people first settled the valley. In the past, its water was used to irrigate nearby rice fields, and the shoreline has changed little over the centuries.

Local people often come here for a short walk or to sit by the water, and the lake appears in haiku and travel sketches from the region. In spring, families gather on the grassy banks for picnics under the cherry trees.

The lake is easy to reach by car or bicycle, with a road running along part of the shore and a few places to park. Walking paths let you approach the water, but some sections are narrow and shared with local traffic.

The lake has no surface outlet, and its water level is kept stable by underground seepage. This makes the shoreline unusually steady, so the same trees and reeds have stood in place for many years.