When the U boats were unleashed on the world's shipping

Kaiten - Warfare History Network

Red Sea attacks: Canada, allies warn Houthi rebels to stop

UK Researchers Find Wreck of a Steamship Sunk by U-Boat in WWI

Canada pledges to work with U.S. over claims to Arctic sea floor

Could the German U Boat Type XXI have decisively altered the course of the Second World War at sea if they had been available in greater numbers? - Quora

The Key Role of the Convoys Naval History Magazine - August 2020 Volume 34, Number 4

The Times archive of First World War articles

U-boat - Wikipedia

MEDIA MONDAY: U-Boat sinks Schooner in the First World War (1917) - Hudson River Maritime Museum