UFO sightings have become a huge part of our culture, spawning movies, books, and art--but never confirmed contact--since a U.S. Air Force balloon crashed in Roswell, NM, in 1947. Though Roswell-related theories have been debunked, it's harder to explain the 1908 Tunguska Event, when an explosion flattened about 770 sq.mi. of taiga. Plus or minus 1, what was the yield in megatons of this impact event?
15 (plus or minus one)