"That's the only reasonable explanation."
"But what could they have been doing?"
"That remains to be seen; but it's our duty to try to find out."
"What's your plan?"
"Well, that scraping noise appeared to me to come from the under side of the hull."
"Then that's the place to look for mischief."
"But how are you going to get at it?"
"Dive over and feel around at about the place where we heard the sound."
"That was on the port side and apparently right under the cabin floor."