When the tide comes in on China's Qiantang River in the city of Hangzhou, something unusual happens. Seawater piles up in the funnel-shaped Hangzhou Bay and roars up the river in a massive wave that can be nine meters high. The tidal wave can capsize boats and upset shipping in the bustling metropolis. Onlookers near the river's banks can suddenly find their lives in danger. But for a peculiar breed of surfers, the chance to ride the world's longest wave is the thrill of a lifetime.

Tidal bore surfers have been cruising river waves like this -- which locals call "the Black Dragon" -- around the world since 1955. They are the marathoners of the surfing world, routinely staying on a single wave for 30 to 45 minutes, and sometimes well in excess of an hour.

Among the icons of the sport is Brazil's Sergio Laus. That's him above on the Dragon last October. Laus is fond of shredding the Amazon's four-meter high bore, or heading up north to catch 7.6-meter high barrels breaking on the Rio Araguari. (Bores aren't common, but dozens of rivers have them.)

As with any sport where you've conquered piranha-, snake- and alligator-infested waters churning with the awesome, life-threatening power of the tides, there's only one thing to do: perform in front of a giant audience.

And there's only one place to do it -- the Qiantang Dragon. Laus and fellow surfers did that, surfing for miles in front of a crowd of close to 500,000 people last fall during Hangzhou's annual festival to honor the massive wave. Did they set a world record? It’s hard to say. There’s a lot of uncertainty about distance whenever a surfer doesn't have a GPS on board. And there is some speculation that Laus set an unofficial record last year during a ride on the Araguari that might have exceeded 14.4 kilometers.

But did they get awesome pictures? You bet.

Jamie Sterling on the Dragon.

Marcos Sifu on the wave.

By Michael Reilly

Editor: Li Hui, Xie Fang