HMS M2 was a Royal Navy submarine monitor completed in 1919, converted in 1927 into the world’s first submarine aircraft carrier.
After the accidental sinking of M1 in 1925, M2 and her sister M3 were taken out of service and reassigned for experimental use. Her 12-inch gun was removed, replaced by a small aircraft hangar, the work being completed in 1927. This could carry a small Parnall Peto seaplane, specially designed for the M2.
In October 1928, a hydraulic aircraft catapult was fitted, to enable the seaplane to take off directly from the deck.
On January 26, 1932, the HMS M2 sank during an exercise in Lyme Bay, Britain. You’d think a submarine would be designed to sink, really.