At 171 m long, the USS Pennsylvania is the biggest submarine in the US Navy. It can dive deeper than a thousand feet, sail for 20 years without refuelling, and remain submerged for up to six months. The vessel carries a crew of 155 men and a deadly nuclear arsenal. Uncover the innovations in six landmark submarines, including the tiny Turtle and colossal German U-boats, that made it possible for engineers to incorporate underwater breathing, torpedos, missile launch systems and stealth technology into ever bigger submarines.