Polar bears, the grand beasts of the arctic, are one of the most fearsome predators on earth. Healthy males can weigh half a ton, stand eight feet tall on their hind legs and possess enough power to get all that mass moving at 25 miles per hour. Few sites in nature will inspire the sense of panic and doom as seeing a polar bear loping across the ice headed your way. They are aggression sheathed in fur, spiked with two inch claws and armed with teeth designed to catch and hold slippery prey. As a rule, few animals survive encounters with these hyper- carnivores. Matt Dyer is that rare exception.