If you ask any climber on the South Side of Everest what the scariest part of the mountain is, most will invariably say that it is the Khumbu Icefall. Ironically enough, this portion of the climb is actually located at relatively low altitude, sitting just above Base Camp. The Icefall is created by the Khumbu Glacier, which begins breaking apart on this section of the mountain. The result, is rather large crevasses that must be negotiated by the climbers in their way up to Camp 1. In order to cross those open areas, a team of Sherpas, known as the Khumbu Ice Doctors, build and continually maintain, a route through the falls using a series of ladders to cross.
So, just how scary is this section of the climb? Check out the video below to get an idea. I've seen these videos, and even the Icefall, for myself, and it never gets any easier to look at.