The Grand Mesa is the largest flat-topped mountain in the world: a basalt-capped plateau covering roughly 500 square miles and running 40 miles across western Colorado, its summit scattered with more than 300 lakes. Colorado Highway 65, the free Grand Mesa Scenic Byway, switchbacks past overlooks from Interstate 70 to Cedaredge, but the real payoff is on foot — the Crag Crest Trail loops 10.5 miles along a ridge strung with alpine lakes.