Tunguska Theory

The world was fortunate that this hit a forest:

A new explanation for a massive blast over a remote Siberian forest in 1908 is even stranger than the mysterious incident itself.

Known as the Tunguska event, the blast flattened more than 80 million trees in seconds, over an area spanning nearly 800 square miles (2,000 square kilometers) — but left no crater. A meteor that exploded before hitting the ground was thought by many to be the culprit. However, a comet or asteroid would likely have left behind rocky fragments after blowing up, and no "smoking gun" remnants of a cosmic visitor have ever been found. [...]

"The sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire," another witness said in a testimonial. Energy released by the blast was later estimated by scientists to be 185 times greater than that of the atomic bomb dropped on Hiroshima in 1945, according to NASA.

Had this hit a densely populated area, millions would have burned in an instant inferno. Today, tens of millions. Entire nations could be wiped out in a few moments.

The researchers estimated that the Tunguska meteor likely measured between 328 and 656 feet (100 and 200 m) in diameter, and hurtled through Earth's atmosphere at roughly 45,000 mph (72,000 km/h).

20 kilometers per second. [Insert Mandarin's line from Iron Man 2.]