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TRY NOT TO SKIM-READ The Tunguska Event, was a powerful explosion that occurred near the Podkamennaya (Lower Stony) Tunguska River in what is now Krasnoyarsk Krai of Russia, at around 7:14 a.m. on June 30, 1908. Although the meteor or comet burst in the air rather than directly hitting the surface, this event is still referred to as an impact.
Estimates of the energy of the blast range from 5 megatons to as high as 30 megatons of TNT, with 10–15 megatons the most likely. The explosion knocked over an estimated 80 million trees over 2,150 square kilometres (830 square miles). It is estimated that the shockwave from the blast would have measured 5.0 on the Richter scale. An explosion of this magnitude is capable of destroying a large metropolitan area. For more Info visit http://en.wikipedia.org/wiki/Tunguska_event
The POD is actually many years prior when a far smaller asteriod(assuming the Tunguska Event was a meteorite impact) collided with the Tunguska Meteorite altering its path so it could potentially hit any location on the earth within 24 hours of OTL strike time.
My question is What is the most significant 20th century altering location it could strike and where would you have it strike?