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Horaisan (Japanese: 蓬莱山), or horaijima (Japanese: 蓬莱島), are terms used to refer to an inaccessible island that generally is part of a Japanese garden, and are often translated as "Treasure Mountain" or "Treasure Island", respectively. The name comes from Horai, a Chinese island important in Japanese mythology.
Japanese garden elements typically include a body of water, several islands, bridges, meditation spots, a few human structures and many types of plants. The horaisan remains unconnected to any other part of the garden: it has no bridges nor does it have any paths or structures. This combination of inaccessibility with the island's beauty symbolizes the realm of happiness not available to mortals.
In contrast to horaisan, nakajima refers to those islands which people may visit. They are connected to each other, and to the mainland, by bridges.