A 1908 explosion in remote Siberia that flattened 2150 km² of forest. Caused by an asteroid or comet that exploded in the air; the largest impact event in recorded history.
From Wikipedia
The Tunguska event was a large explosion of between 3–50 megatons TNT equivalent that occurred near the Podkamennaya Tunguska River in Yeniseysk Governorate, Russia, on the morning of 30 June [O.S. 17 June] 1908.