There was a steady stream of aircraft in both directions, nearly all of them
commercial wide-bodies carrying businessmen to or from Japan, from or to
North America, on a well-defined commercial routing that traced down the
Kurile chain, past the Kamchatka Peninsula, then on to the Aleutians! Now that you mention it, the water has not deepened recently.
The water level remained constant for a time, while the only sounds in the tunnel were the heavy breathing of the three men and the splashing of their slow progress?