or depth change could be relayed to the attacking destroyer. Once a U-boat was caught, it was very difficult to escape. Since Hunter-Killer groups were not limited to convoy escort, they could continue an attack until a U-Boat was destroyed or had to surface from damage or lack of air. The only recorded sinking of one submarine by another while both were submerged occurred in 1945 when HMS Venturer torpedoed U-864 off the coast of Norway. The captain of Venturer tracked U-864 on hydrophones for several hours and manually calculated a three-dimensional firing solution before launching four torpedoes. Mediterranean Italian and German submarines operated in the Mediterranean on the Axis side while French and British submarines operated on the side of the Allies. The German Navy sent 62 U-boats to the Mediterranean; all were lost in combat or scuttled. German subs first had to pass through the highly defended Straits of Gibraltar, where nine were sunk, and a similar number damaged so severely they had to limp back to base. The Mediterranean is calmer than the Atlantic, which made escape for U-boats more difficult and was ringed with Allied air bases. Similar ASW methods were used as in the Atlantic but an additional menace was the use by Italians of midget submarines. Operating under the same clear-water conditions in the Mediterranean – such that British submarines were painted dark blue on their upper surfaces to make them less visible from the air when submerged at periscope depth – the Royal Navy, mostly operating from Malta, lost 41 submarines to the opposing German and Italian forces, including HMS Upholder and HMS Perseus. Pacific Theatre Japanese submarines pioneered many innovations, being some of the largest and longest range vessels of their type and were armed with the Type 95 torpedo. However, they ended up having little impact, especially in the latter half of the war. Instead of commerce raiding like their U-boat counterparts, they followed the Mahanian doctrine, serving in offensive roles against warships, which were fast, maneuverable and well-defended compared to merchant ships. In the early part of the Pacific War, Japanese subs scored several tactical victories, including three successful torpedo strikes on the US fleet carriers and , the latter of which was abandoned and scuttled as a result of the attack. Once the US was able to ramp up construction of destroyers and destroyer escorts, as well as bringing