He made the first ascents of some of the hardest sport routes in the U.S., including Kryptonite with grade of 5.14c/d and Flex Luthor with a grade of 5.15a, at the Fortress of Solitude, Colorado. In January 2015, Caldwell and Kevin Jorgeson completed the first free climb of the Dawn Wall of El Capitan. At the time, their 19-day ascent was considered by some as the hardest successful rock climb in history.