The thermal effects of the "Little Boy" nuclear bomb if it was dropped on New York City.
Not that it's particularly likely, but as long as nuclear bombs exist, there's the chance - however slim - that one might go off somewhere near you. This little Google Maps overlay might be a bit morbid, but it's also pretty fascinating. It shows you the heat, pressure and fallout spread of a range of different nuclear bombs detonating anywhere in the world. It's particularly sobering to get a sense of the scale of the devastation caused by the Hiroshima and Nagasaki bombs in World War 2 - and then see how tiny those bombs are compared to the USSR's enormous Tsar Bomba, the biggest nuke ever detonated.