SG.
I don`t think anything is classified, more history. Risky missions are not usually planned, they just happens for various reasons. The first risky mission was probably the first dive after being mothballed for several years after WW2. During the first years of the cold war and NATO we thought it would be OK to introduce the 3 VII Cs in our submarinefleet. The VII Cs were stowed away in a Norwegian fjord with very limited drawings and instruction material. The German navy did not exist, thus no possible German info. Old submariners got the job to figure out the details and I am told the first tests were carried out in the harbour of Trondheim with the biggest available harbour crane hook secured to the hull prior to the test dive. Incident occurred and risky situations ,not mission, arise. I have experienced two. During a work up period with fairly fresh crew we should carry out a routine dive and some how the main dieselair inletvalve was not shut, luckily the inletvalve into the engineroom was, but the whole airduct from the conningtower to the engine room was filled with water app. 5 tonnes, creating an unintended deep crash dive which didn`t stop till we were well below 100 meters. A similar situation occurred during a testdive after an annual docking. You always have an accurate weight calculation prior to such a dive to be sure you have the right ballast. No 1 being the responsible officer made a mistake and at the dive we went down like an elevator. The CO very quickly took action and ordered to blow all the tanks but we were so heavy so it took quite some time before we stopped at record depth where we were "hanging" some nervewrecking seconds before we slowly started to climb to the surface. However surfacing by blowing air and not using hydroplanes at depths exceeding 100m creates a jet effect by the expanding air and you shoot out of the surface like a torpedo and hope no surface vessels are in the neighbourhood. Other wise I was in charge of the longest schnorchel research mission ever at that time in our navy. 28 days submerged which I wouldn`t put in the category risky.
Tore