Using evidence, Jerry doesn't see the tunnel, what does he see?

From the text:

Jerry dived, shot past the school of underwater swimmers, saw a black wall of rock looming at him, touched it, and bobbed up at once to the surface, where the wall was a low barrier he could see across. There was no one visible; under him, in the water, the dim shapes of the swimmers had disappeared.

He could see nothing through the stinging salt water but the blank rock.