It turns out that these things don't have to hit the ground to do a lot of damage. Now, in 1908, over Siberia, near the Tunguska region -- for those of you who are Dan Aykroyd fans and saw "Ghostbusters," when he talked about the greatest cross-dimensional rift since the Siberia blast of 1909, where he got the date wrong, but that's okay. It was 1908. That's fine. I can live with that. Another rock came into the Earth's atmosphere and this one blew up above the ground, several miles up above the surface of the Earth. The heat from the explosion set fire to the forest below it, and then the shock wave came down and knocked down trees for hundreds of square miles, okay? This did a huge amount of damage. And again, this was a rock probably roughly the size of this auditorium that we're sitting in. In Meteor Crater it was made of metal, and metal is much tougher, so it made it to the ground. The one over Tunguska was probably made of rock, and that's much more crumbly, so it blew up in the air. Either way, these are tremendous explosions, 20 megatons. Now, when these things blow up, they're not going to do global ecological damage. They're not going to do something like the dinosaur-killer did. They're just not big enough. But they will do global economic damage, because they don't have to hit, necessarily, to do this kind of damage. They don't have to do global devastation. If one of these things were to hit pretty much anywhere, it would cause a panic. But if it came over a city, an important city -- not that any city is more important than others, but some of them we depend on them more on the global economic basis -- that could do a huge amount of damage to us as a civilization.