The ARA San Juan is a Argentine Navy diesel-electric attack submarine built in 1983. CNN reported, the submarine has been missing since November 15th with a crew of 44 sailors on board.
Per CNN, the vessel was last spotted in the San Jorge Gulf a few hundred miles off the coast of Patagonia. It was traveling north from a base in Tierra del Fuego to another in Mar del Plata. According to the BBC, NASA has contributed a P-3 explorer aircraft to the search effort.