http://en.wikipedia.org/wiki/Tunguska_event

The Tunguska explosion felled an estimated 80 million trees over 2,150 square kilometers (830 sq mi). An overhead satellite view, from nearly a century later, centered at 60°55′N, 101°57′E (near ground zero for this event) shows an area of reduced forest density with a clearly visible, irregular clearing of somewhat less than one square kilometer in area.