Touching the void

What does "touching the void" mean - esp. when related to mountaineering?

Well, I couldn't find a meaning. But I am guessing that it means a near-death experience. The "void" meaning whatever is waiting for us. The great unknown.

Anyone have the answer to this one?

My understanding is that it means to achieve the unachievable, experience the impossible. Something along those lines. I think that, in this usage, 'The void' is a vacuum, such as Outer Space, which cannot be touched, but to actually do so surpasses normal human experience.