Merlin, according to Geoffrey of Monmouth in Historia Regum Britanniae, might well have been the son of a demon who impregnated a mortal woman who was a nun. Yet, Tennyson in his Idylls of the King describes ocean waves bringing the infant Arthur to the shore, where Merlin carries him to safety.
The cave, a very real one, at the base of the cliffs beneath Tintagel Castle fills with water when high tide comes in. Certainly, this is a mysterious dark cavern, nowadays a place where scuba-divers search for shells and fossils. It runs clear through the headland, constantly being eroded by the sea.
In Emily’s Shadow, Merlin’s cave is accessible from the top of the cliffs through a hole that has been boarded due to the danger of falling. It leads to an opening above the sea, where water trickles onto sharp volcanic rocks protruding from the seabed down below. Perhaps, as surely as Merlin could shape-shift, so too could he conjure dark openings to suit his purposes. Of course, now, the hole at the top of the cliffs is no more, and the trickling stream is a fault line, running down the face of the cliff, pointing to many small caves where witches might hide.