I've always heard/read that it was from protein and organic waste... no clue on the accuracy of that statement though. I always just accepted it and moved on, though I would be interested in an actual scientific explanation.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry