Here's how breaking down "subterranean" helps you understand its meaning:
* Sub: As you correctly identified, this prefix means "under" or "below".
* Terra: This comes from the Latin word for "earth" or "land".
Putting it together, "subterranean" means "under the earth" or "underground".
This is a great example of how knowing prefixes and roots can help you decipher unfamiliar words!