We know that the United States, France, the United Kingdom, Russia, India, Pakistan, and China have nuclear bombs, and more recently Israel announced possession of one while Iran and North Korea are always trying to build them, but no one knows for sure if they have succeeded or not.
Nuclear bombs, also called atomic bombs, have widely varying yields. The known technologies are nuclear fusion and nuclear fission.
Fission is the more known one; besides being destructive, they cause uranium and plutonium contamination. Fusion, with the Hydrogen Bomb as the prime example, would theoretically generate extremely superior power to the other, but in a clean way.
Nuclear bombs contaminate the environment where they explode in such a way that it becomes unusable for hundreds or even thousands of years. An exception would be the Neutron Bomb, which has the capacity to destroy living beings without damaging buildings, a considerable military advantage.
