http://en.wikipedia.org/wiki/Tsar_BombaTsar Bomba is/was the world's largest bomb, and had a power of 50 loving megatons (originally planned to be 100). Only one working example of the bomb was built and detonated 4500m above the ground, and boy.
The mushroom cloud went up to about
67 loving kilometers, the impact force went around the Earth
3 times. The light flash could be seen from
a mile. The people
270 km away from the site not only saw the explosion through sunglasses, they
felt heat. Here's the destruction tally from a small village
400km away: wooden houses
kaput, stone houses had their roofs
caved in, and their windows and doors
blasted off. The detonation site was a
plain flatland, no cliffs, no stones, no hills, no nothing.
info from http://www.verybigsite.ru/bomba.html (warning: russian language)...ouch.