The Unusual Sinking of Submarine U-28
During the First World War, it was typical for German submarines to use their deck guns to sink merchant vessels, rather than using torpedoes.
On 02 September, 1917, the small German submarine U-28, 971 tons displacement and only 212 feet long, attacked the British merchant ship Olive Branch, 4,649 tons.
The submarine fired torpedoes which only damaged the merchant ship, but did not sink her. So, the German submarine surfaced in order to use her deck guns to finish off her victim.
What the Captain and crew of U-28 did not know was that Olive Branch was carrying a full load of ammunition and the initial torpedo hit had started a fire in the cargo hold.
While the submarine was firing her deck guns, the ship's cargo exploded, sinking the submarine.
Some reports have said that the explosion was so severe that a truck was thrown into the air and came crashing down on the submarine. While this may not be true, there is no doubt that U-28 was sunk by the explosion of her victim.
There were no survivors from the submarine.
Source: Great Britain's National Maritime Museum: