'The Thousand Caves', the hidden halls of Thingol and Melian on the river Esgalduin m Doriath; see especially 58, 106-8, 111-2, 125, 130, 134, 145, 155, 200, 203, 208, 217, 222-6, 229, 243-7, 252, 267, 269, 286-91