funny you would say that, thats exactly what came to mind when i thought about it (Megalodon)... maybe its not really extinct like they think.. i suppose it could be a 25 foot Great White, but those are extremely rare, if ever seen.. although, a few years ago off the coast of Australia a 10 footer was bitten in half nearly by what they say was a 20+ foot "monster shark"... noone saw that shark either, just the one that had chunks out of it...

ill see if i can find the article..