Tidal island

A tidal island is a piece of land that is connected to the mainland by a natural or man-made causeway that is exposed at low tide and submerged at high tide. Because of the mystique surrounding tidal islands, many of them have been sites of religious worship, such as Mont-Saint-Michel with its Benedictine Abbey. Tidal islands are also commonly the sites of fortresses because of their natural fortifications.
List of tidal islands.
Europe.
Iceland.
Grótta in Seltjarnarnes, the Capital Region
United Kingdom.
Wales.
43 (unbridged) tidal islands can be walked to from the UK mainland.