Questions & Answers about Das Erz, das sie suchen, liegt in der Mine.
Since Erz is a mass noun, how would I say "the mining of ore" using the grammar from this lesson?
Because Erz (ore) in this generic sense is an unmarked mass noun with no article or adjective to show the genitive case, you must use von + Dative instead of the genitive. You would say der Abbau von Erz rather than der Abbau Erzes.
Why is the relative pronoun das in the accusative case here?
The second das is a relative pronoun referring back to Das Erz. It takes the accusative case because it acts as the direct object of the verb suchen inside the relative clause (they are seeking it).
Could I say nach dem sie suchen instead of just das sie suchen?
Yes. The verb suchen can take a direct accusative object (etwas suchen) or be used with the preposition nach + Dative (nach etwas suchen). If you used nach, the relative pronoun would change to Dative: Das Erz, nach dem sie suchen.
Why use liegt (lies) instead of ist (is) to describe where the ore is?