Global Descents made a big splash in the rafting game by being the first company to offer amateurs access to remote rivers like India's raging Zanskar and Madagascar's remote Matsiatra. Never particularly profit-oriented, the company's founders have a tendency to tackle the nearly impossible with a sort of rebellious glee.

"We're river runners much more than businessmen," says owner Matt Gontram. "We're all about running rivers, so if someone calls me and says, Hey, I've always wanted to go do this river in Argentina, – or anywhere – we'd say, Let's go down and do it."

Gontram's next project, the Usumacinta River, is a rapid-studded fire hose that marks the border between Mexico and Guatemala. Once the territory of Sandinistas, it has been off-limits for years, even though the conflict in Chiapas quieted down long ago. Though the river's rapids generally don't exceed Class III, the adventure quotient on the Usumacinta is high. It's one of the biggest rivers on the continent, flowing at 40,000 to 100,000 cubic feet per second (by comparison, the Colorado River does between 8,000 and 25,000 cfs through the Grand Canyon), and gigantic eddies and whirlpools flip boats like pancakes. But what is most interesting lies on shore. Rafters camp on empty beaches larger than football fields, then explore travertine pools and waterfalls the color of Windex, before checking out ancient Mayan ruins abandoned deep in the jungle.

More Information: Global Descents' first raft trip down Mexico's Usumacinta River runs about $1,750. The outfitter also has upcoming expeditions on the Zambezi in Zambia, the Siang River in India, and the Futaleufu in Chile.