Aokigahara (青木ヶ原), also known as the Sea of Trees (樹海, Jukai), is a forest on the northwestern flank of Japan's Mount Fuji thriving on 30 square kilometres (12 sq mi) of hardened lava laid down by the last major eruption of Mount Fuji in 864 CE. The western edge of Aokigahara, where there are several caves that fill with ice in winter, is a popular destination for tourists and school trips. Parts of Aokigahara are very dense, and the porous lava absorbs sound, helping to provide visitors with a sense of solitude.The forest has a historical reputation as a home to yūrei: ghosts of the dead in Japanese mythology. In recent years, Aokigahara has become internationally known as "the Suicide Forest", one of the world's most prevalent suicide sites, and signs at the head of some trails urge suicidal visitors to think of their families and contact a suicide prevention association.