The M2 was one of four M series submarines. In addition to the traditional torpedo armament, these submarines all carried a single 12-inch turret mounted gun in front of the conning tower, a gun big enough to knock holes in a battleship. In 1927 the M2's gun turret was replaced with a hanger for a seaplane and a launching catapult on the forward deck, becoming a unique submarine aircraft carrier.
A surfaced submarine is vulnerable, so the crew of the M2 was constantly to surface, launch the seaplane and dive again as quickly as possible. It is thought that one of these drills was to cause the sinking of the M2, with the submarine taking on water through open hanger doors and sinking rapidly.
On the morning of 26 January 1932 the M2 was taking part in submarine exercises in the eastern region of Lyme Bay. Several ships sighted the M2 diving, but a submarine diving was not an unusual sight and attracted no special attention until the M2 failed to return to Weymouth that evening.
A good charter boat skipper will drop a shot line right across the conning tower of the M2, an ideal place to start and finish a dive. As you descend your first sight of the submarine will be a selection of masts and periscopes rising from the conning tower.
Immediately towards the bows is the seaplane hanger. At the back is a bank of fine silt that is invariably stirred up by the first few divers to venture inside, so if you want to have a look in the hanger getting there first is a priority!
From the hanger I like to follow the launch rails towards the bows. The bows are a straight vertical edge with a full covering of anemones. The main points of interest here are the outer openings for the four 18-inch torpedo tubes. These are slots that go right through the bows with square backs where the actual tubes are inside the bow of the submarine.
From the bows follow either side of the submarine to the stern. It is worth rising slightly to inspect the bow hydroplanes before putting on some speed because the sides of the submarine are almost featureless except for the a dense carpet of hydroids and the occasional clump of dead men's fingers. Nevertheless, keep an eye out for discarded bits of salvage equipment, particularly amidships, and the occasional small opening could be home to some interesting marine life.
At the stern the two prop shafts, keel and rudder are again covered in anemones, fed by the strong current that whips round the end of the wreck when the tide is running. This can also be a good location for large shoals of bib and poor cod.
Having seen the stern there is nothing left to do but ascend to deck level by the rear hydroplanes and make a quick swim back to the conning tower. On the way you will pass the "disappearing" 3-inch gun platform, with the actual gun slotted flush with the surface of the platform in its lowered position. Back at the conning tower you can use any remaining time to explore more thoroughly amongst the periscopes and other fittings.