What landform act as a natural barrier between north africa and the rest of the continent?

Which two words in this excerpt from the count of monte cristo can be used as synonyms to determine the meaning of the word grotto? dantès entered the second grotto. the second grotto was lower and more gloomy than the first; the air that could only enter by the newly formed opening had the mephitic smell dantès was surprised not to find in the outer cavern. he waited in order to allow pure air to displace the foul atmosphere, and then went on.