The Great Kanto Earthquake of September 1st, 1923, caused a 200m wide cave-in, damming Ichikizawa and creating Lake Shinsei. Even after almost 100 years, the "lake surface," "collapsed area," and "dammed area" can still be seen. It was registered as a national monument on March 26th, 2021. September of 2023 will mark 100 years since its creation.
15-minute bus ride from JR Hadano station.