Divernet
The M2 is a one-of-a-kind submarine aircraft carrier which sank off Portland in 1932 during routine exercises (see panel). It is 90m long, still in one piece, and sits upright on the seabed at a depth of about 35m, with the bows pointing north-north-east.
As a wreck tour it gave me a bit of a problem - what can you say about diving an intact submarine with no holes to explore Except, perhaps, that it is a unique vessel and can be covered in one dive with minimal decompression, which is probably why it is one of the most popular wrecks in Lyme Bay.
A good charter-boat skipper will drop a shotline right across the conning tower of the M2, an ideal place to start and finish a dive (1). As you descend, your first sight of the submarine will be a selection of masts and periscopes rising from the conning tower.
The towers deck, which will be your point of arrival on the sub, hosts some large and colourful anemones and is often surrounded by large shoals of fish. But as it is shallower here than elsewhere on the wreck, save having a good look around until the end of your dive.
First, swim along the top of the seaplane hangar and recovery winch (2), to have a look inside the hangar (3). At the back is a bank of fine silt that is invariably stirred up by the first few divers who venture inside, so try to be first there if you can!
All other openings in the submarines pressure hull have been sealed, presumably during the salvage attempts, so this is the only opportunity to go inside the wreck.

From the hangar I like to follow the launch rails (4) towards the bows. Beside the rails, at a depth of 30m, is a grated decking the crew would have used when recovering the seaplane. Gaps in the rails are often home to crabs and conger eels. I have even seen the occasional lobster hiding here. Forward of the rails is a large single bollard, or capstan (5), and various small deck fittings and valve openings that are home to tompot blennies.
The bows (6) are a straight vertical edge with a full covering of anemones. There is a small anchor winched in tight against the starboard bow.
The main points of interest here are the outer openings for the four 18in torpedo tubes (7). These are slots with square backs that go right through the bows. The actual tubes are inside the bows of the submarine. These holes seem to serve as a designer condominium for conger eels.
Below the bows, the sandy seabed has been scoured by the tide to give the deepest point of the dive at 35-36m - a good hiding place for crabs and lobsters.
From the bows, follow either side of the submarine to the stern. It is worth rising slightly to inspect the bow hydroplanes (8) before moving quickly towards the stern following the sides of the submarine, which are almost featureless except for a dense carpet of hydroids and the occasional clump of dead mens fingers. Nevertheless, keep an eye out for discarded bits of salvage equipment, particularly amidships (9). The occasional small opening could be home to some interesting marine life.
At the stern the two propshafts, keel and rudder (10) are covered in anemones, fed by the strong current that whips around 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, ascend to deck level past the rear hydroplanes (11) and make a quick swim back to the conning tower. On the way you will pass the disappearing 3in gun platform (12). The gun itself is not visible, being in its lowered position flush with the surface of the platform.
Back at the conning tower you can use any remaining time and air to explore more thoroughly among the anemones and marine life of the periscopes and other fittings.


CREWS SPEED WAS THEIR DOWNFALL  
In 1927 HMS M2 became the worlds first undersea aircraft carrier - a submarine carrying a small two-seater seaplane in a watertight hangar, writes Kendall McDonald. The seaplane, which had folding wings, was launched by catapult off a runway on the deck. When it landed near the sub, it was hoisted on board and into the hangar by a specially designed small crane.
The M2s crew was proud of the speed with which it could launch the little floating plane. Rising to periscope depth, the submariners would check around for enemy ships, surface, open the hangar door and catapult the plane off on its flight. They were constantly trying to beat their own speed record.
It was this speed which was to kill all the crew. The M2 dived at 10.11am during exercises off Portland on 26 January, 1932, and disappeared. On 29 January all hope was officially abandoned for the 60 crew aboard.
It took eight days to find her. The first divers discovered that the hangar door was open with the plane still inside. The hangar had apparently been opened while she was still underwater.
A salvage operation retrieved the plane, but after 11 months and a total of 1500 dives involving 26 Royal Navy divers, the Admiralty had to admit defeat over the submarine. On 8 December, 1932, the M2 was left to rest forever on the seabed off Portland.


Divernet
Divernet
Divernet







TIDES: Slack water is between 3 and 4 hours after high water Portland. On neaps the tide also drops to a diveable state (but not fully slack) 2 to 3 hours before high water Portland.
GETTING THERE: For Weymouth: from Dorchester, take the A354 to Weymouth, continue along the back of the harbour, turn left just before the fire station. From there it depends which boat you are meeting. For Portland: take the A354 past Chesil Beach, turn left to the south side of Portland Harbour as the road starts to climb. Dive centres are in the old dockyard area.
DIVING AND AIR: From Weymouth: boats can be chartered with Pat Carlin (01305 787155), Chris Caines (0976 766169), Paul Pike (0966 143489), Grahame Knott (0966 242460) or the Old Harbour Dive Centre (01305 760888), which also supplies air. From Portland: Fathom and Blues (01305 826789) and Old Harbour Divers at Aquasport (01305 861000) for air and boats, Brian Charles (01305 822846) and the Portland Dive Centre (01305 820870) for boats.
LAUNCHING: Slips are available at Weymouth, Portland, West Bay and Lyme Regis. Harbour and launch fees are payable.
ACCOMMODATION: Many B&Bs and small hotels. Campsites are usually smart and a bit expensive. The Sailors Return (01305 773377) on Weymouth Quay offers a convenient B&B. Fathom and Blues has accommodation above its dive centre. Also Skin-Deep Diving (01305 782556) and Glendinning Lodge (01305 760393). Weymouth and Portland Tourist Information (01305 785747) has a complete list.
QUALIFICATIONS: You must be able to dive to 35m and ideally do a few minutes decompression. Technical diving is not required, but it is an ideal depth for extending bottom time with a nitrox mix.
FURTHER INFORMATION: Admiralty Chart 3315, Berry Head to Bill of Portland. Ordnance Survey Map 194, Dorchester, Weymouth and surrounding area. Dive Dorset, by John and Vicki Hinchcliffe. Shipwreck Index of the British Isles, Volume 1, by Richard and Bridget Larn. The Divers Guide to Weymouth and Portland Area, Weymouth and Portland BSAC. On the web, try www.deepsea.co.uk.
PROS: An intact submarine that can be fully dived without excessive decompression. Well serviced by hard boats and RIBs. Air and nitrox easily available.
CONS: Slack water essential. Shot can easily drag clear of wreck.

Thanks to Alex Poole, Chris Caines and Axel Forryan for their help.