Once a along time ago, about 16 millions years, our blue planet was infested with 52 foot long-40 ton white sharks. These massive predators, called Megladon, were themselves feeding on critters three to four times that size which had plated armor on the sides.
Man, don't you wish you had a titanium cage and a time machine right about now?
Anyway, we know these beasties existed because, like so called modern white sharks, they shed their teeth faster than you can pump nickels into a Vegas slot machine.
Those teeth fell to the bottom of the ocean where divers are still finding them today. One of our divers who joined at at Isla Guadalupe in 2003 showed us an 8 inch Meg tooth that was still sharp enough to cut steak, we know because we tried. Here's a video of a Meg tooth hunt sucess: