existential threat

But if this awful noise proceeds from a cataract--if, so to speak in plain English, this vast interior ocean is precipitated into a lower basin--if these tremendous roars are produced by the noise of falling waters, the current would increase in activity, and ...

I felt the steel plates tremble at the fastenings of the bolts; its bars bent, its partitions groaned; the windows of the saloon seemed to curve under the pressure of the waters. And this firm structure would doubtless have yielded, if, as its Captain had said...