And that river is still used now for recreation.But anyway…let's analyze the problem. It wasn't the mining itself that caused such massive destruction.

MALE PROFESSOR: It was what happened after the copper ore was extracted from the mines.It was a process called heap roasting.

MALE PROFESSOR: Copper ore contains sulfur, and heap roasting was a way to burn away the sulfur in the copper-so they'd be left with something closer to pure copper.

请问老师这里的it指代什么？ 如何理解 it was what happened after/before这类句型？

这句话可以改写成 It was the thing which happened after the copper ore was extracted from the mines. 进一步改写 The thing happened after the copper ore was extracted from the mines. 吗？ 如果可以 It was what happened 这种句型是为了强调吗？