I was wondering if the Tunguska Event would be brought up as I remember seeing a History Channel documentary on it a couple years back. Sure enough.............
http://www.foxnews.com/science/2013/02/15/russian-meteor-blast-recalls-massive-108-tunguska-event/
That 1908 blast, which was later attributed to a comet or asteroid fragment, is generally estimated to have been about 10 megatons. No injuries were reported, but some 80 million trees over 830 square miles were leveled in the blast known as the “Tunguska event.”
The explosion near the Podkamennaya Tunguska River on June 30, 1908, reportedly flattened some 500,000 acres of Siberian forest. Scientists calculated the Tunguska explosion could have been roughly as strong as 20 megatons of TNT, or roughly 1,000 times more powerful than the atomic bomb dropped on Hiroshima, Space.com reported.
The longstanding theory regarding the cause of the event is a cosmic impact from an asteroid or comet. In the last decade, researchers have conjectured the event was triggered by an asteroid exploding in Earth's atmosphere and measuring roughly 100 feet wide and 617,300 tons in mass — more than 10 times that of the Titanic.