It was sort of a freak accident.
I was on a Grand Canyon exploration adventure with a group of friends from New York City. The whitewater rafting was very tame, and periodically the raft would stop along the rocky embankment so that passengers could get out and walk around.
During one stop, some of us went hiking up an embankment that wasn't very steep. As I put my foot on one of the large rocks, it came loose. I am guessing the rock had been there for tens of millions of years or so, and I just picked the wrong geologic moment to step on it. The boulder, which weighed somewhere between 500 and 1,000 pounds, came loose and fell on top of me, and together we fell several feet down.
It would have been pretty ugly but for the fact that the weight of the boulder came to rest mostly against a smaller rock and much less against me. Still, I was completely pinned under the boulder, and it was slowly sliding further against me, which is a situation I definitely recommend avoiding.
The Grand Canyon has very interesting metamorphic and igneous layers — according to the tourist narrative we got the next day, which I was paying particularly close attention to. I think my boulder came from something called the Vishnu Schist — but they are less interesting when you are sandwiched between them.
Fortunately, there were a lot of people around who went to get help. Unfortunately, it turned out that in the course of falling, the rock and I had fallen into the lair of a rattlesnake. Who, somewhat understandably, went into full strike mode. (This really happened.) So the rescuers, who included some level-headed river guides and some highly snake-phobic city people, had to retreat and come around a different route to get at the boulder. It took six of them to cantilever it so that I could wriggle out.
My favorite moment was when my good friend Anne, who reliably yelp-screams when she sees an ordinary house spider, ran to the boat to see if anyone had a gun. I think she intended to shoot the rattlesnake (or maybe to scare it by brandishing the gun?), though maybe it was to finish me off if the situation didn't look like it was going to end well.
I hadn't seen the James Franco movie 127 Hours (and definitely don't intend to), so I never thought about chewing off any of my limbs. But this is why in the future I am going to try to stay away from the outdoors. And I recommend you do the same.