Running rim-to-rim at the Grand Canyon -- or rim-to-rim-to-rim -- is a challenge more and more runners are doing. While those runners revere the experience, their growing number is creating problems that might soon be addressed.

To hike the Grand Canyon is a challenging, beautiful experience. To run from one rim of the canyon to another -- and sometimes back -- is almost a super-human effort that requires ultra fitness and the ability to withstand extreme conditions. Yet die-hard trail runners love the test and the chance to see more of the canyon than most visitors ever do. Still, the growing number of runners in the canyon is leading to some problems that will have to be addressed. My story for ESPN.com's Endurance Sports section: http://espn.go.com/sports/endurance/story/_/id/10870741/endurance-sports-grand-canyon-runs-beautiful-spiritual-problematic