The forest surrounding Mount Koya (Koyasan) in Wakayama is a sacred mountain and temple complex founded by the monk Kobo Daishi. It is the location of his mausoleum and is surrounded by the Okunoin Cemetery, Japan's largest.

Kūkai (空海; 27 July 774 – 22 April 835), also known posthumously as Kōbō Daishi (弘法大師, "The Grand Master who Propagated the Dharma"), was a Japanese Buddhist monk, civil servant, engineer, scholar, poet, artist and calligrapher who founded the esoteric Shingon school of Buddhism. He travelled to China, where he studied Tangmi (Chinese Vajrayana Buddhism) under the monk Huiguo. Upon returning to Japan, he founded Shingon—the Japanese branch of Vajrayana Buddhism. With the blessing of several Emperors, Kūkai was able to preach Shingon teachings and found Shingon temples. Like other influential monks, Kūkai oversaw public works and constructions. Mount Kōya was chosen by him as a holy site, and he spent his later years there until his death in 835 AD.

Lit by thousands of lanterns, Oku-no-in is the mausoleum of the monk that founded the complex. According to tradition, the lights have been burning constantly since his death more than 1,000 years ago. It is around that mausoleum that the cemetery is located. Known not only for its size and significance, but for its unique headstones, Okunoin is filled with giant spaceships, cups, and other strange monuments erected for the former employees of astronautical and coffee companies. A special monument was built by a pesticide company to commemorate all of its insect victims.

(Atlas Obscura)

Okunoin Cemetery panorama

In 2004, UNESCO designated Mount Kōya, along with two other locations on the Kii Peninsula, Yoshino and Omine; and Kumano Sanzan, as World Heritage Sites "Sacred Sites and Pilgrimage Routes in the Kii Mountain Range".

The complex includes a memorial hall and cemetery honoring Japanese who were imprisoned or executed for committing atrocities during World War II. (See oped: Mount Koya sites exemplify 'parallel universe' where war criminals are martyrs)

Note: As there are many temples with identical names, Chinese and Japanese temples are traditionally given additional "mountain names". These are no geographical designations. There is no mountain called Kōya-san in Japan.

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