Merlin's Cave

Merlin's Cave is a natural sea tunnel beneath Tintagel Castle in Cornwall, England, connecting Tintagel Haven on the east side of the island to West Cove on the west. At low tide, it is possible to walk from one entrance to the other along a sandy floor. At high tide, the tunnel becomes impassable as it fills with seawater from end to end. The cave is approximately 100 metres (330 ft) long and was formed by marine erosion along a thrust plane between slate and volcanic rock. The cave is named after the legendary wizard Merlin, a central figure in Arthurian mythology. This association arises from its proximity to Tintagel Castle, which Geoffrey of Monmouth identified in the 12th century as the site of King Arthur's conception at the behest of Merlin.

Cave data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).