Learn German: Der Ozean ist sehr tief, was wirklich beeindruckend ist. - The ocean is very deep, which is really impressive.

Here, was is an indefinite relative pronoun. It doesn't refer to a single noun, but to the entire idea in the first clause (the fact that the ocean is very deep). When you want to say 'which' and refer back to a whole sentence, you must use was.

Could I use das instead of was here?

No, in standard German, you must use was when referring back to a complete sentence. If you used das, a German listener would try to find a specific neuter noun you are referring back to in the first half of the sentence, but there isn't one.

Why is the second ist at the very end of the sentence?

The word was introduces a relative clause. In German, relative clauses are a type of subordinate clause, which means the conjugated verb (ist) gets kicked to the very end of its clause.

The word beeindruckend looks like a verb. Is it an adjective here?

Yes! It comes from the verb beeindrucken (to impress). By adding a -d to the end of the infinitive, you create the present participle. These function just like adjectives in German, similar to adding '-ing' in English, though here it translates best as 'impressive'.