Decima MAS

The Decima Flottiglia MAS (Decima Flottiglia Motoscafi Armati Siluranti/10th Anti-submarine Motorboat Flotilla, which was also known as Decima MAS, La Decima, or Xª MAS) was the Royal Italian Navy's special forces unit. It undertook a number of different tasks including

• The use of frogmen to attach limpet mines to sink ships that were in harbour
• The use of crewed self-propelled torpedoes (SLC (Siluri a Lenta Corsa) or Miaile (Pig)) to deliver frogmen to targets inside harbours
• The use of explosive motorboats (MTM (Motoscafo da Tourism Modificato)) to sink enemy shipping

The Decima MAS mounted some remarkable operations including:

• 25th March 1941: The wrecking of the British heavy cruiser HMS York in Suda Bay, Crete, when she was hit by an MTM

HMS York in 1938.

• 18th December 1941: The sinking of the Royal Navy battleships HMS Queen Elizabeth and HMS Valiant by limpet mines placed on the ships' bottoms in Alexandria by frogmen using Miaile crewed, self-propelled torpedoes. Both ships were put out of commission for several months whilst repairs were undertaken.

HMS Queen Elizabeth at anchor, surrounded by anti-torpedo nets.

HMS Valiant.

• 13th July 1942: The sinking of four merchant ships (SS Meta, SS Empire Snipe, SS Shuma, and SS Baron Douglas) in Gibraltar Harbour by limpet mines laid by twelve frogmen who swum across from the interned tanker Olterra which moored in Algeciras, Spain.

The Italian tanker Olterra. The frogmen who carried out the attack on 13th July – and later nine Miaile crewed, self-propelled torpedoes operated by Decima MAS and used in subsequent attacks on ships in Gibraltar Harbour – were able to enter and leave the ship undetected via a special hatch that had been cut into her side below the waterline.