Although his other faculties were deadened partially by mental and physical exhaustion, his hearing was uncommonly acute, concentrating in itself the strength lost by the rest.

The sound was peculiar, half a swish and half a roll, and although not loud it remained steady.

Ned listened a long time, and then, all at once, he recognized its cause.
He was under the sea, and it was the rolling of the waves over his head that he heard.

He was in one of the famous submarine dungeons of the Castle of San Juan de Ulua.

This was the hospitality of Cos and Santa Anna, and it was a hospitality that would hold him fast.