The largest, according to the Guinness Book of Records, is the Qiantang River tidal bore in China. It can reach a height of 29.2 feet (8.9 meters) and a speed of 25 miles an hour (40 kilometers an hour).
It is locally known as the Silver (or Black) Dragon. Each year a tidal-bore-watching festival is held around the 18th day of the Chinese lunar calendar in September, when the bore is at its largest.
The Severn River in the United Kingdom has over 250 bores a year. The phenomenon has been known to carry surfers upriver for up to six miles (ten kilometers).
The Lupar Benak bore in Sarawak, Malaysia, nearly drowned the British writer Somerset Maugham during a visit to the Rajah in 1949.
But one of the most fascinating bores is the pororoca in Brazil.
Not everyone runs for cover when the big roar is heard on the banks of the Amazon River. In 1997, for the first time, a group of intrepid surfers grabbed their boards to see if they could ride the wall of water.
But tidal bore surfing is not for amateurs. The Silver Dragon in China is so dangerous that no one has managed to remain upright on it for more than 11 seconds.
The pororoca has its own set of challenges, according to Serginho Laus, a professional Brazilian surfer and journalist who has conquered many tidal bores.
"The wave is very powerful and can destroy anythingtrees, local houses, islands and sweeps up wild animals, like snakesthe anacondaalligators, spiders, piranha, and even jaguar," said Serginho, who once broke his back trying to surf the pororoca.
Add to those risks the candiru (Vandellia cirrhosa), a small parasitic fish that can swim up a person's urethra, dig in its spines, and feast on its host's blood. The invader can only be removed by surgery.
So why do Serginho and his fellow surfers brave the muddy and dangerous waters of the world's tidal bores?
"You can see the jungle and ride the longest wave in the world more than 10, 20, 30, 40 minutes," he says with barely contained enthusiasm. "The best pororoca is in MaranhaoMearim River perfect with tubes!"surfer slang for the rounded hollows formed by breaking waves.
In April Serginho will head for the Amapathe longest and most dangerous pororocato try to break Adilton Mariano's record ride of 36 minutes. In the meantime, the sport of bore surfing is becoming more and more popular, with charters providing trips to even the most inaccessible Amazonian rivers.
Which means surfers are now facing a new and completely unexpected danger in the jungles of Brazilovercrowding.
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