An investigation into the rate of degredation of wood-pulp products in an Australian landfill has determined that porn magazines — with their coated, glossy pages — outlast other types of printed matter and will be the last printed items in the landfills to rot away. Fark concludes that this means that "porn will be this civilization's gift to the next civilization."
He found that magazines, newspapers and old bits of wood thrown away up to 46 years ago were in almost perfect condition, with pornography lasting the best of all.
He displayed a 1979 copy of the men's magazine Playboy which was in near mint condition, and said its thick wax coating could be the reason for its longevity.
Update: Not only does porn rot slowly, but it also contains lots of kaolin, a mildly radioactive clay used to coat paper and render it glossy. Because so much porn is printed on clay-coated stock, it is more radioactive than other forms of printed entertainment. This chart of the radioactivity of a Playboy mag is particularly excellent. (Thanks, Mike!)