Extensive research has been done into this disease.

对这种疾病已进行了广泛研究。

A great deal has been done internally to remedy the situation.

内部已经采取了很多措施来挽救局面。

I find it ludicrous that nothing has been done to protect passengers from fire.

我认为没有采取任何措施来保护乘客免受火患是荒唐的。

It has been done by a scientific experiment.

这是科学实验的的结果。

But it has been done before.

不过，此前已有人这样做了。