Which 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.