World's Largest Tidal Bore, Qiantang River, China

Tidal bore
The river and bay are known for the world's largest tidal bore, which is up to 9 metres (30 ft) high, and travels at up to 40 km per hour (25 miles an hour). The tide rushing into the river from the bay causes a bore usually from 5 to 15 ft (1.5–4.6 m) high, which sweeps past Hangzhou and menaces shipping in the harbor. It is so dangerous that, until recently, no one attempting to surf it has managed to remain upright for more than 11 seconds.
Firstime, in September 2008 a group American surfers convinced the Chinese government to allow them to surf a section the river. Personal watercraft were used to safely transport the surfers in and out the tidal bore.