The Crystal Cave is a mysterious cave where Merlin is currently imprisoned. Its whereabouts are as yet unknown. 
Real World Background
The notion of Merlin being imprisoned or asleep in a cave somewhere is a familiar one, at least ever since the story of his imprisonment by Nimue began. Malory locates this cave in Cornwall, but other legends have provided other sites for it. Welsh tales have Merlin asleep in a secret "House of Glass" on Bardsey Isle, off the northern coast of Wales, surrounded by thirteen magical treasures, or in a cave near the town of Carmarthen in southern Wales, where he was reputedly born. Mary Stewart in her Merlin trilogy popularized the notion of Merlin's cave being a crystal one (the idea goes back at least to a poem by Edwin Muir, published in 1937), and identified it as the one near Carmarthen. Whether Merlin's Crystal Cave in the Gargoyles Universe is at any of these locations or not is as yet unknown.