The Soviet Union once tested a nuclear bomb, that shattered all records. They called it the Tsar Bomba. If it were used over New York, it would produce fatalities of approximately 7.6 million people and injure millions more. Yes, that is with a single bomb and in just a few seconds. In comparison, the Hiroshima bomb would kill around 263,000 New Yorkers. Above is a picture of a dummy Tsar Bomba.
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Information sources:
http://nuclearsecrecy.com/nukemap/
http://www.aasc.ucla.edu/cab/200708230009.html
Photo Credits / Sources:
By User:Croquant with modifications by User:Hex (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY-SA 2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons