For another, Earth is an active planet. Air, water, tectonics... these are all sources of erosion, and after a few hundred thousand — or hundred million — years, craters fade away. If it weren't for those factors, the surface of the Earth would look like the surface of the Moon, saturated with craters. Nonetheless we do have some techniques to get absolute ages for craters (as opposed to relative, where you can say this crater is older than that one). But even then it can be tricky to get good numbers. Cases in point: Two of the youngest, large-ish, and well-preserved craters on Earth have just been re-evaluated, and the ages we