Learn Italian: Qual è la profondità massima di questo cratere? - What is the maximum depth of this crater?

Massima is the feminine form of massimo, the irregular absolute superlative of grande (big/great). While you could technically say la profondità più grande, using massima is the standard, natural way to express "maximum" or "greatest" when talking about limits, measurements, and abstract concepts.

Since the lesson covers age too, could I use massima to mean "oldest" for a person?

No, massimo refers strictly to "maximum" or "greatest" in size, limit, or degree. To talk about the "oldest" or "eldest" person, you would use the irregular form maggiore or simply il più grande.

Why is there no apostrophe in Qual è?

It is a very common trap! Qual is actually a complete, valid word on its own in Italian, so it doesn't need an apostrophe when placed next to è. Writing qual'è is a spelling mistake that even native speakers sometimes make.

How do we know the gender of profondità?

Nouns ending in an accented -tà are almost always feminine. As a handy bonus, they are also invariable, meaning they do not change their endings in the plural. "The depths" would simply be le profondità.