Gunkanjima is an abandoned island located in Nagasaki. Surrounded by a sea wall, you will find an entire abandoned city with huge concrete buildings. More info on https://offbeatjapan.org/gunkanjima.
Aokigahara (青木ヶ原), also known as the Suicide Forest or Sea of Trees (Jukai), is a forest that lies at the northwest base of Mount Fuji in Japan. The forest contains a number of rocky, icy caverns, a few of which are popular tourist destinations. Due to the wind-blocking density of the trees and an absence of nearly all wildlife, the forest is known for being exceptionally quiet.