In 2007, paddler Dan West, then an undergraduate at Oregon's Lewis and Clark College, examined this question in a report called (what else?) "The Physics of Kayaking Down Waterfalls." Kayak durability isn't an issue—you could drop today's polyethylene boats from 1,000 feet—but human durability is. West crunched the numbers by kayaking small waterfalls with an accelerometer, which measures G-forces, and plugging his findings into physics formulas. Based on this, he figured the limit might be around 186 feet, a mark that was matched in 2009 when Tyler Bradt paddled off Washington State's 189.5-foot Palouse Falls and walked away with only a sprained wrist.

Still, West probably wasn't far off, judging by what happens to people who jump off the 245-foot Golden Gate Bridge to commit suicide. They hit the water at approximately 85.5 miles per hour, a jolt that can smash internal organs, break bones, and sever the aorta. Kayakers have one advantage over jumpers—they hit bow first, in pools aerated by waterfalls—but a cushion of bubbles can soften the blow only so much.

Will people keep pushing this crazy record? Bradt, who was likely going 75 when he landed at Palouse, thinks he's probably finished. "I'd have to have a lot of motivation to do something bigger," he says. Meanwhile, Rush Sturges, a top paddler who filmed Bradt's descent, predicts that someone will try a 200-foot waterfall within two years. (One likely candidate is Iguaçú Falls, on the border of Brazil and Argentina, which has several drops over 200 feet.) West thinks kayakers should seek out other thrills. "Somebody is going to get seriously hurt," he says.