A hundred years ago today, in Tunguska, Siberia... something happened. Nobody is quite sure what, exactly, but the leading theory is that an asteroid slammed into the Earth's atmosphere and exploded, causing a blast wave that flattened trees over half a million acres, or 2000 square kilometers.
The asteroid may have been a rocky asteroid roughly 30 meters in diameter with a mass of 560,000 metric tons. Or it may have been smaller, only 20 meters across with a mass around 200,000 tons, since the trees were already weakened by disease and would not have been as hard to knock down. Or it may not have been an asteroid at all, since no asteroid remnants have been found. Some researchers believe it may have been an iceball, a fragment of a comet, that exploded into a fine mist and thus left no trace.
Some people have other ideas, as they always do when there is no evidence. Some people think an explosion of natural gas from deep under ground caused the blast wave. It's also possible—but unlikely—that a micro black hole zipped through the Earth, or that a chunk of antimatter from space impacted the atmosphere. Some people even believe that Nikola Tesla may have been experimenting with a "death ray" that he dismantled after he learned of its destructive power. And finally, some conspiracy theorists believe that the impact was caused by the crash landing of a UFO. Of course none of these ideas has much credence, as there is no evidence to support them. And as we know from Occam's Razor, the simplest explanations tend to be right.
In any event, the Tunguska Incident highlights how much we still don't know about the universe around us, and how exposed we are here on spaceship Earth. If a blast like the Tunguska Incident happened today over a major metropolitan area, thousands or millions of people could be killed, and we'd still be left scratching our heads, wondering what in the heck just happened.