Answer:
Opening, cavern
Step-by-step explanation:
Synonyms are words that have identical or nearly identical meanings. An example of synonyms are words present and gift. They mean the same thing and are interchangeable.
When synonyms are used in the same context and we aren't familiar with one of them, we can figure out what it means based on the other one. Words used in the given passage that help us determine the meaning of the word grotto are opening and cavern.