I served on WWII diesel boats in the early '50s. As a Sonarman, part of my duties were to maintain little smoked glass slides that were inserted into a device to measure the temperature of the water. On maneuvers, we'd take a test dive in the area, down to say, 200 feet. The skipper would watch for a jig in the scribed marking that would indicate a marked change in the gradient - in the Caribbean, it was usually 150 feet.
When we found the target, usually a convoy, we'd go in and shoot the Hell out of them, and when the destroyers came after us, we go below that gradient and let them conduct their futile searches. They never did find us.
A sidebar to that was that another part of my duty was to inscribe the Lat and Long, dip those used slides in varnish and send them off to some scientists in Washington. For all I know, they are still there in some basement.