One of the world's most impressive natural sights is the Qiantang River's tidal bore. This massive wave, which can reach up to 30 feet in height and achieve speeds of 25 mph, draws plenty of gawkers to watch it smash into the shore. While there are other tidal bores around the world, the Qiantang River is perhaps the best one to view. Evidence of it dates back at least 3,000 years.
For a tidal bore to occur, a river's mouth has to be shallow and funnel-like. The river also needs to have extreme changes in water levels between high and low tides. Because of the narrow nature of the river mouth, the water becomes confined at high tide. This water meets the shallow water left behind, begins to move very quickly forming waves and then eventually breaks, forming a bore.
What makes the Qiantang River's tidal bore so special is the Hangzhou Bay, where the river empties. The bay is basically just a giant triangle. While it measures more than 60 miles across at places, the gap narrows to less than 13 miles when it meets the Qiantang. The result is a very narrow funnel for all the water to flow through, creating the hourglass look seen in the video.
Other factors, including the weather, can play a part in the bore's strength. In 2013, a typhoon made landfall in eastern China, and the result were waves in the Hangzhou Bay that exceeded 60 feet and injured at least 30 people.