I imagine large craters are something that are still being discovered. Especially when they're very old and eroded, but some faint outlines can still be made out, and they just find the right rock in them I guess,(reply>), Statistically, since so much of the planet is water, there must have been a bunch of big astroids that hit water rather than land, so there's a good chance there's at least one bit of a meteorite sitting at the bottom of the ocean just waiting for someone to find it.