The limestone massif of the Rock of Gibraltar is riddled with caves - its ancient name, Calpe, means hollow - and it was here that archaeologists focused their efforts to find sites of Neanderthal occupation. Ten such sites have been discovered so far, of which the most important are five caves on the eastern side of the Rock: Ibex Cave, high up on the east side, which was only discovered in 1975 due to being buried under the wind-blown sands of the Great Gibraltar Sand Dune, and four sea caves near sea level on the south-eastern flank, Boathoist Cave, Vanguard Cave, Gorham's Cave and Bennett's Cave.