A cave is an opening in rock, for example in the side of a cliff or a mountain, and can be large or small. In prehistoric times, people lived in caves.

1. …a large collection of caves known as the Blue Grotto.
2. The cliffs are riddled with caves and almost every one shows some sign of ancient habitation.

A cavern is a very large cave, usually underground.

3. Gradually the underground caverns fill up with deposits.

Areas below ground in buildings are called cellars or basements. Cellars are often used for storing things that are no longer wanted or that are not used very often.

4. I decided to return the unwanted books to the cellar.

A basement is designed to be used regularly or lived in. Department stores often have basements, and some restaurants and bars are in basements.

5. He stopped outside the door leading down to the basement, where Dr Firmius lived.