"The future of free climbing is in the big mountain regions of the world," such as the Karakoram range, Chin said. The challenge is so unimaginable, he said, "because you're adding altitude and cold and severe weather."

And unlike Yosemite, where a rescue (or a latte) is within range, these areas are incredibly remote.

"There would be no chance of a rescue," Hill said. "You have a hard time just breathing and getting supplies there."

The east face of the Great Trango Tower, which reaches about 20,000 feet, tops the list because of its near-vertical drop -- the word's biggest. Even with ladder-like devices to move through the harder sections, people have died trying to climb and descend it.