Given the following context:  During ancient times, Gibraltar was regarded by the peoples of the Mediterranean as a place of religious and symbolic importance. The Phoenicians were present for several centuries, apparently using Gorham's Cave as a shrine to the genius loci of the place, as did the Carthaginians and Romans after them. Excavations in the cave have shown that pottery, jewellery and Egyptian scarabs were left as offerings to the gods, probably in the hope of securing safe passage through the dangerous waters of the Strait of Gibraltar.The Rock was revered by the Greeks and Romans as one of the two Pillars of Hercules, created by the demigod during his tenth labour when he smashed through a mountain separating the Atlantic and the Mediterranean. According to a Phocaean Greek traveller who visited in the sixth century BC, there were temples and altars to Hercules on the Rock where passing travellers made sacrifices. The Spanish later symbolised the importance of the Pillars of Hercules with a heraldic device consisting of a pair of columns with a scroll wrapped around them – a symbol that became the $ sign and the related Portuguese cifrão ().To the Ancient Romans, Gibraltar was known as Mons Calpe, a name perhaps derived from the Phoenician word kalph, "hollowed out", presumably in reference to the many limestone caves in the Rock. It was well-known to ancient geographers, but there is no known archaeological evidence of permanent settlements from the ancient period. According to the Roman writer Avienus, the ancient Greek traveller Euctemon recorded that thirty stadia separate [the Pillars of Hercules]; [Euctemon] says that they bristle with woods all over and are always unwelcoming to seamen. Indeed he says that on those are both temples and altars to Hercules. He says that strangers sail there by boat to make offerings to the gods and depart hot foot thinking it wrong to linger ...  answer the following question:  What was the meaning of the word that the Ancient Romans name for Gibraltar was derived from?
A:
hollowed out