The floating torii at Miyajima is a solution to a religious problem, not an aesthetic choice.
The problem
The island itself was the object of worship. Mount Misen and the island as a whole were held sacred, and the ground was regarded as too holy to be built on or casually walked on.
A shrine was nonetheless required.
The solution
Build it over the sea.
The buildings stand on piles over a tidal flat, connected by covered corridors, so that at high water the complex and its great torii stand in the water and the structure does not rest on sacred ground at all.
Worshippers approached by boat, passing through the torii from the sea, which is the direction it was designed to be entered from, and the reason it stands out in the water rather than at a land approach.
At low tide the flat drains and the torii can be walked to, which is how most visitors now see it.
Birth and death
Historically neither birth nor death was permitted on the island.
The pregnant were removed to the mainland to give birth. The dying were taken across. There was no cemetery, and those who had attended a death observed a period of mourning off the island before returning.
This follows from kegare, ritual pollution, in which birth and death are the principal sources of defilement, and it was applied here with unusual rigour because the island itself was the deity.
Some of these restrictions persisted into the modern period. Precisely when each was relaxed is not something this entry states, because the commonly repeated dates are not well sourced.
Kiyomori
A shrine is traditionally dated here to the late sixth century, but the complex as it stands is owed to Taira no Kiyomori.
As governor of the province he rebuilt it in 1168 on a courtly scale and made it his clan's tutelary shrine, lavishing on it the resources of a man who was becoming the most powerful figure in Japan.
That is why a provincial island shrine has the architecture of a Heian palace, the corridors, the proportions, the stage over the water. The Taira presented sutra scrolls to it that survive as national treasures.
The clan was annihilated at Dan-no-ura in 1185. The shrine outlasted them.
The deities and the torii
The shrine holds three sea goddesses, daughters of Susanoo, appropriate to an island in a shipping strait.
The present great torii dates from 1875, is about sixteen metres high, and stands on its own weight rather than being set into the seabed, weighted with stones packed into its box beams. It underwent extensive restoration between 2019 and 2022, during which it was entirely scaffolded.
UNESCO inscribed the shrine in 1996.