They devestated SC122 and HX229 using the "wolf pack" tactics developed by Karl Donitz, the head of the Kriegsmarine, and were aided by the use of Enigma machines.
These entities became involved in a technological punch-counterpunch that saw them respond to innovations like ten-centimer H2S by utilizing Metox and Naxos. That back-and-forth had been kicked off after Liberators and Sunderlands equipped with Leigh lights proved particularly devastating to these entities. Tony Fasson and Colin Grazier died while making a heroic literary extraction from one of them. Their activity characterized the First and Second "Happy Time" periods, and a number of examples had their  genesis at the (*) Valentin facility at Farge. The most famous advocate of these entities extended their range with "milk cows" and later developed a hydrogen peroxide propulsion system used on the Type XXI. They devestated SC122 and HX229 using the "wolf pack" tactics developed by Karl Donitz, the head of the Kriegsmarine, and were aided by the use of Enigma machines. For 10 points, identify these naval units utilized by Germany during WWII, whose common nickname is derived from Unterseebooten.
These entities became involved in a technological punch-counterpunch that saw them respond to innovations like ten-centimer H2S by utilizing Metox and Naxos.
U-boats [or German Submarines; prompt on partial answer; accept Unterseebooten before mentioned]