HMS M1 was the British submarine armed with a 12-inch deck gun during World War I.
M1 began life as one of the M-class submarines, which were designed to carry a large 12-inch naval gun. The weapon was mounted forward of the conning tower and was intended to attack surface ships without requiring a torpedo hit.
The concept was unusual because submarines normally relied on torpedoes or smaller deck guns. M1’s gun could fire a very heavy shell, but the submarine had to surface to aim and operate it, exposing itself to enemy fire.
After the war, M1 was lost in 1925 after being struck by the coastal steamer Vidar in the English Channel. M2 was later converted into a submarine aircraft carrier, while M3 became a mine-layer. The M-class boats are remembered for their experimental weapon design.