Lake Shinsei, Natural lake in Hadano and Nakai, Japan

Lake Shinsei is a landslide-dammed lake on the northern Ōiso hills, straddling Hadano City and Nakai Town in Kanagawa Prefecture. It covers 13,000 m², has a perimeter of about 1,000 m and an average depth of 4 m, reaching 10 m at its deepest. No stream flows in or out; the water connects to surrounding systems through groundwater. A drum bridge crosses a narrows dividing the main western basin from the smaller eastern one, and the cliff left by the collapse stands on the east shore.

On 1 September 1923 the Great Kanto Earthquake triggered a landslide on the slope of Ichikizawa in present-day Hadano; the debris dammed the valley's uppermost stream and water gathered in the hollow. The slip plane was weathered Hakone volcanic ejecta, the Tokyo pumice layer about 66,000 years old, turned to halloysite clay. The collapse scarp survives as the cliff on the lake's east side, and the lake was first called a sunken pond before local volunteers named it Shinsei in 1924.

The lake's name was chosen around 1924 by local volunteers; a claim that seismologist Terada Torahiko coined it is false, since he surveyed the site only in 1930. Near the bus stop at the entrance stands a memorial tower for two schoolchildren swept away by a landslide at Minezaka, about 300 m north, during the 1923 earthquake. A stone monument at the shore records a haiku by Terada: "Mountains split, a pond is made—water striders."

A 15-minute bus ride from JR Hadano Station brings you to the lake. Two free parking lots are available for those arriving by car.

The lake's name was chosen around 1924 by local volunteers, though a persistent story credits seismologist Terada Torahiko, who surveyed the lake only in 1930. A stone monument at the bus stop entrance commemorates two schoolchildren swept away by a landslide at nearby Minezaka during the 1923 earthquake.