The death of King Arthur is very mysterious, like most of the Arthurian legends. Most of them are romantic poetry of the late middle ages, while the whole story happend, if it really happened, in the 6th century. So after half a millennium the people had no idea what really happened, instead they made up a story which represents the taste of the time it was written.
One legend, which is told, is the Cave Legend. King Arthur and his Knights have taken refuge in a mystical cave, where they will rest until the time Britain needs them most.
This motive is very similar to the story of Emperor Barbarossa. But in this case the are two possible places, where this mysterious cave may be. To be honest, one of them is actually a small cave.