The Itsukushima Shrine is a Shinto shrine dating back to the 6th century. The buildings that you can view at present date back to the 16th century. The shrine was devoted to worship of the goddesses(!) that the warlord who funded it, Kiyomori, owed thanks to for his successes.
The shrine was built up over the bay so that it would appear to be floating on the water at high tide. Most famous is the floating torii of Itsukushima Shrine that acts as a gate to the shine. Like the rest of the buildings, it only appears to be floating at high tide. At low tide you actually can walk underneath it. There has been a gate here since 1168, although this torii only dates back to 1875.