Mount Misen is the highest mountain on Miyajima Island at 535 m. Miyajima became holy because, unlike the other islands in the area, it has huge boulders on it at the top of Mt. Misen, and the Shinto religion reveres striking natural features. There are two main routes up the mountain, the Daisho-In route and the Momijidani park route. I would recommend going up one and coming down the other to be able to take in all the sights.
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