While stealth remains the defining characteristic of modern submarines, speed can still make the difference between intercepting an adversary, escaping danger, or reaching a strategic position first. Interestingly, many of the fastest submarines ever built emerged during the Cold War, when the Soviet Union and the United States competed to push underwater engineering to its limits. Some designs prioritized sheer speed at the expense of noise and operating costs, while modern submarines increasingly balance speed with stealth. Here are seven of the fastest submarines ever built. Project 705 Lira (Alfa class) – 41 knots (76 km/h) If one submarine embodied Soviet engineering ambition, it was the Alfa class.