Four mountain climbers who have climbed Mount Everest are now taking a shot at reaching the summit again. Peter Whittaker, Ed Viesturs, Dave Hahn and Melissa Arnot began their expedition to the top of the world's tallest mountain on March 30. Some of the climbers are going for records on their way to the summit. Dave Hahn is going for his incredible 11th Everest ascent, and Melissa Arnot is attempting to be the first American woman to reach the summit without extra oxygen.