Underwater Speed Records
Official underwater speed records -- usually achieved by military submarines -- are not kept due to the secrecy surrounding the capabilities of these warships. But in 1965, the USS Albacore, a Gato-class submarine, was clocked at 33 knots (38 mph), an unofficial underwater speed record. Claims of higher speeds have been made by submarine manufacturers but have not been officially measured.
Russia's Akula-class submarine allegedly can travel at 35 knots (approximately 40 miles per hour) submerged, while the Alfa-class submarine it replaced was said to reach 44.7 knots (approximately 51.4 miles per hour) for short periods.