Cavernous describes something so large and hollow inside that it reminds you of a cave. A cavernous room has such high ceilings and so much empty space that sounds echo when you talk, just like they would in an actual cavern deep underground.

When you walk into a cavernous gymnasium or warehouse, you feel small because of all that open space around and above you. The word captures both the size and the emptiness: a cavernous building might have hardly any furniture in it, making it feel even more vast and hollow.

You might read about a dragon's cavernous mouth in a fantasy story, suggesting a mouth so huge and dark it looks like you could walk inside it. Or someone might describe cavernous eye sockets on a skull, emphasizing how deep and hollow they appear.