The single fact that makes polar bears the most terrifying critters out there in the wilds today is not that they're huge and strong and fast and amphibious and invisible on infrared and not at all averse to preying on humans when there's nothing else around (and there's not much around in the arctic wastelands they call home). No, the scariest thing about polar bears is the following:
On the ice, the only visible part of a polar bear (to the eye and to infrared both) is its black nose. Polar bears know this, and cover their noses with one paw while stalking their prey, rendering themselves completely invisible.
Now that's scary.
The moral of the story? Don't ever mess with a polar bear. Ever. They are more badass than you can ever hope to be (and they just evolved that way, without going to yossarian's school or taking lessons from thefez!) And if you ever see me covering my nose with one hand, know that I'm being polar bear-stealthy at you and be afraid.
Boy, am I in a silly mood. Sorry about that. Go vote up zgirll's excellent factual writeup on these magnificent monsters while you're downvoting me into oblivion.
Note (26 January 2002): Razhumikin, who admits that he has perhaps seen a few too many polar bear documentaries in his day, says the eyes of polar bears are also visible to regular and infrared light both. Still, their eyes are smaller than their noses, and they cover their noses! That's so scary!