Finding a submarine means searching for a vessel designed to remain out of sight. NATO says that concealment, combined with the ability to carry torpedoes and ballistic missiles, makes submarines a serious security threat to allies.
The documentary follows ships and aircraft trying to pinpoint submarines beneath the surface. NATO says allied forces use sonar-equipped ships, helicopters and maritime patrol aircraft to locate submarines and, if necessary, neutralise them.
Allies train to hunt submarines three times a year, according to NATO. The alliance says each body of water presents different conditions, making regular training necessary across the areas where its forces may operate.
