WATCH: Dangerous, freak tidal wave on China's Qiantang River

About this time each year, folks in China's eastern-shore-hugging Zhejiang Province gather to watch the weird spectacle of a hulking wave rolling up the Qiantang River. The wave is what's known as a tidal bore, a disturbance created by the gravitational pull of the moon and the funnel shape of the Hangzhou Bay. The bore's crest can rise to 30 feet tall and, at a speed of 25 m.p.h., sounds like "thousands of horses running."

The Chinese have been gleaning entertainment from the Qiantang wave, which is the largest bore in the world, for thousands of years. Some people even like to surf on it. On Wednesday, however, the bore decided to entertain itself at the Chinese's expense. Massive amounts of ocean water racing into the river fed the wave's growth until it was a frothing, rumbling monster that certain reports say reached 90 feet tall. The out-of-control bore smashed into a dike in Haining City and tossed spectators around like so many rubber duckies; some tourists were sucked down an embankment and more than 20 people were injured. (Check out these heavy photos of the waterlogged mayhem.)

The freakish size of the bore, the largest in nearly a decade, was probably due to the influence of a nearby storm system called Nanmadol. The typhoon had undertaken a havoc-sowing journey from the Philippines, where it killed at least eight people including two children buried under an "avalanche of rubbish." By late August, Nanmadol had swelled into a Category 4-strength cyclone and was throwing powerful winds and ocean swells at the Chinese coast, amplifying the Qiantang bore's strength to dangerous levels.

You can watch the bore run wild in the below video – that reporter was lucky to not be standing closer to the river. Tidal bores are not unique to the Qiantang, occurring on the Kampar River in Indonesia, the River Severn in Wales and in Alaska's Cook Inlet, among other places. Bore lovers might consider watching this footage, apparently narrated by Michael Caine, of the notorious Wiggenhall Wave on England's River Great Ouse. It's anything but... boring.

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