Turnagain Arm is one of only about 60 bodies of water worldwide to exhibit a tidal bore. The bore may be more than six feet high and travel at 15 miles per hour on high spring tides. Turnagain Arm sees the largest tidal range in United States, with a mean of 30 feet (9.2 m), and the fourth highest in the world, behind Bay of Fundy (11.7m), Ungava Bay (9.75m), and Bristol Channel (9.6m). The ocean's natural 12-hour 25-minute tidal cycle is close to Turnagain Arm's natural resonance frequency, which then reinforces the tide similar to water sloshing in a bathtub. Tidal fluctuations in the main body of Cook Inlet, while not as extreme as the shallow and narrow Turnagain Arm, regularly reach 25 feet (7.6 m) and exhibit currents in excess of 5 knots (9.3 km/h) at full tidal flow.