This past Labour Day, as beachgoers up and down the eastern seaboard of the U.S. enjoyed a sunny holiday at the shore,

Erik Petkovic was in the darkened cabin of R/V Explorer some 40 miles off the Virginia coast.

Peering into a video monitor linked to a remotely operated vehicle (ROV) some 400 feet below, he suddenly exclaimed, “That’s it! There it is!”

The object that caused his excitement was the wreckage of SM U-111, the last World War I-era German submarine to be discovered in U.S. waters.

The sleek tube of riveted iron had been part of the Unterseeboot (U-boat) fleet that struck terror in Allied sailors.

After the war an American crew brought the captured submarine across the Atlantic in a daring solo voyage that required navigating the icy waters where R.M.S. Titanic had sunk seven years earlier.

There are more than a dozen German U-boat wrecks from both World Wars in U.S. waters, and the submarines hold a special fascination for shipwreck divers like Petkovic.

He’s one of a small number of so-called “technical divers” who explore depths wildly exceeding the standard 120-foot limit observed by recreational divers.