The 'cave' is actually a rock-cut Byzantine necropolis containing seven sarcophagi, one of which is considerably smaller than the rest (in the Sura 18 version it's six men and their dog - hence the smaller tomb).
Although it's well and truly off the tourist trail, the cave is wonderfully atmospheric and well worth a visit. The contrast between the beauty and tranquility inside the cave and the chaotic road outside is amazing. The door is normally locked but the custodian of the adjacent mosque will be happy to let you in. If you want to ask directions or instruct a taxi driver, the local name is "Ahl al Kahf".