The Schnellboot is laying not far from the entrance of the Grand Harbour in 67m. Therefore Advanced trimix certification is required for the dive. Usually currents are not strong here, however be prepared and stay very close to the shotline. The visibility can be easily disturbed since the bottom is silty. Look for the open torpedo tubes, they give perfect opportunities for photographers.
The outside cover of the Second World War German S-Boats were mahogany casing with an internal hull of light metal, which as wrecks, have rotted away. Therefore the structure of the boat is well visible. The S-31 had three Daimler Benz MB502 diesel engines with 3 propellers for a total of 4800 BHP and a maximum speed of 38 knots. Usually equipped two 533 mm torpedo tubes and two 20mm guns. It could hold a crew of 24 men. At the time of its loss, the S-31 had 2 Italian officers as observers, who were among the survivors.
The S-31 belonged to a sub-group numbered from S-30 to S-37. All of these had been built by Lurssen at Vegesack, Germany and were launched in 1939. During the afternoon of 9 May 1942, the Luftwaffe indicated that the HMS Welshman would arrive in Valletta before dawn on 10 May 1942. Therefore the German 3rd MTB Flotilla carried out an operation to intercept the mine layer as it approached Valletta harbour from the northeast.
Four MTBs (S-54, S-56, S-57 & S-58) would wait for the HMS Welshman off the St. Thomas area to attack as the boat approached up the southeastern coast of Malta. Three other MTBs (S-31, S-34, and S-61) first laid a minefield stretching from Sliema point in order to cover the side in case the HMS Welshman approach Valletta via the northwest instead. Thereby creating a fresh minefield which the British would have had no time to clear. After laying the minefield, the three MTBs were to head east to add their torpedoes in the attack against the HMS Welshman.
The 7 MTBs left Augusta at 2200 on 9 May 1942 and at 0414 on 10 May, the three MTBs began laying the minefield in the shape of an isosceles triangle. The minefield consisted of 20 FMC contact mines, 6 explosive buoys and 2 cutting buoys. The minefield was ready in 7 minutes at 0421. The three MTBs had to regroup and head eastwards to search for the HMS Welshman. But suddenly a minute after the mine-laying operation had stopped, the S-31 exploded. It is possible a mine, had cut loose from its moorings, rose to the surface and drifted into the S-31. The ship sunk at 0438 and the S-61 managed to save 13 survivors, including C.O. Lt. Heinrich Haag. 13 men were lost.