The German Type 212 is a highly advanced design of non-nuclear submarine (U-Boat) developed by Howaldtswerke-Deutsche Werft AG (HDW) for the German Navy. It features diesel propulsion and an additional air-independent propulsion (AIP) system using Siemens proton exchange membrane (PEM) hydrogen fuel cells. The submarine can operate at high speed on diesel power or switch to the AIP system for silent slow cruising, staying submerged for up to three weeks without surfacing and with no exhaust heat. The system is also said to be vibration-free, extremely quiet and virtually undetectable.