When you think about it, the Earth has had a rough time of it. You'd never guess by looking, but over its 4.5 billion year history it's been hit a zillion times by asteroids, from dust motes up to something Mars-sized that whacked us and ended up forming the Moon. You'd expect the planet to be scarred, pockmarked, from all this. But the Earth is restless, and shifty. Even though there probably isn't a square meter of the planet that hasn't been hit, over millions of years erosion has taken its toll, wiping out all but a few of the craters. Some are obvious enough, like Barringer Crater in Arizona. I've been there, and it's amazing. But the one that wiped out the dinosaurs is 100 or more kilometers across, and most of it's underwater. It takes some pretty sophisticated technology to even know it's there. So what's a crater aficionado to do? Why, go to the Impact Field Studies Group homepage, of course. This group is like Audubon for extinction-level events; they travel around doing field studies at impact sites. The site has pictures (the panorama above is from their page; click it for a better image and a bunch more), descriptions, and this is way cool, an Excel spreadsheet listing 540+ suspected impact sites across the planet. Want a nightmare? Sort them by crater diameter. Yikes.
