This one always stuck with me (maybe don't read if you're squeamish):

On 20 July 1981, 24-year-old David Allen Kirwan from La Canada, California, was driving through Yellowstone's Fountain Paint Pot thermal area with his friend Ronald Ratliff and Ratliff's dog Moosie. At about 1:00 P.M. they parked their truck to get out and take a closer look at the hot springs; Moosie escaped from the truck, ran towards nearby Celestine Pool (a thermal spring whose water temperature has been measured at over 200°), jumped in, and began yelping.

Kirwan and Ratliff rushed over to the pool to aid the terrified dog, and Kirwan's attitude indicated he was about to go into the spring after it. According to bystanders, several people tried to warn Kirwan off by yelling at him not to jump in, but he shouted "Like hell I won't!" back at them, took two steps into the pool, and then dove head-first into the boiling spring.

Kirwan swam out to the dog and attempted to take it to shore; he then disappeared underwater, let go of the dog, and tried to climb out of the pool. Ratliff helped pull Kirwan out of the hot spring (resulting in second-degree burns to his own feet), and another visitor led Kirwan to the sidewalk as he reportedly muttered, "That was stupid. How bad am I? That was a stupid thing I did."

Kirwan was indeed in very bad shape. He was blind, and when another park visitor tried to remove one of his shoes, his skin (which was already peeling everywhere) came off with it. He sustained third-degree burns to 100% of his body, including his head, and died the following morning at a Salt Lake City hospital. (Moosie did not survive, either.)