The cave at the foot of the cliff under Tintagel Castle is called Merlin’s Cave, although since the tide fills it I doubt Merlin lived here. It did make me think of a passage in a book by Storm Constantine though.
The cave is really a tunnel: you can come out the other side. It would be easy to get trapped here; in North America there would probably be warning signs and fences and people to make sure the tourists were safe. here you’re on your own.