When you hear about Russian subs and warships in UK waters, spying on data cables or testing UK defences, they are probably part of GUGI – the Russian acronym for the Main Directorate of Deep Sea Research. Since the end of the Cold War, GUGI has avoided the defence cuts imposed elsewhere, allowing it to pour millions into submarine development. They are effectively Russia’s Special Forces of the sea, capable of carrying out extreme deep-sea operations.

Source: GUGI: Russia’s covert military unit spying, sabotaging and operating on UK’s doorstep