During the height of the Cold War there were 50,000 nuclear weapons in active inventory, so had they been used the world would have been devastated beyond belief... and yet mankind probably would have survived somehow. The US made a doomsday machine out of Project Pluto but only tested up to 2/3rds scale model before abandoning the idea of an unmanned fleet of bombers flying at Mach 3 sustained for weeks on a nuclear ramjet engine hitting multiple targets while contaminating all areas in which it flew! The USSR claimed to also have a doomsday machine with its "Dead Hand" system designed to launch its nuclear arsenal even if all communications had been severed with its armed forces and with all cities destroyed. It is unclear if Russia still operates such a system. But there are 3 more bombs that are scarier- the Cobalt Bomb (C-Bomb) known as the Devastator, the AMB (Anti-Matter Bomb) known as the Annihilator, and the UFB (Unified Field Bomb) which is the Unthinkable Weapon. The C-Bomb would range from 50-100 MT and contaminate the earth for hundreds of years. It is 100% possible to build this. The AMB is at least 100s of years away at current tech levels but any unforeseen advances in Anti-Matter Explosives Research at CERN under German control could yield a weapon contained in an EM bomb casing (which the Germans have already designed)! Such a bomb detonated in the atmosphere would light it on fire and destroy most life on the planet. The Germans state that only 3-4 bombs would kill humanity! And lastly, the UFB is unthinkable technology. A bomb detonated over NYC could rip it from its foundations and instantly teleport it over LA and be dropped! Millions of tons of concrete, steel, and glass dropped on another city. It could also teleport targets into the sea or anywhere on earth. A seemingly sci-fi weapon, Einstein's uncompleted Unified Field theory was used in WW2 with the Philadelphia Experiment that teleported the USS Eldridge from port to port and back. It was a disaster. Those that jumped overboard mid-teleport vanished into hyperspace forever. Others bonded on the molecular level with the ship's deck and bulkheads, being fused to them. The USN got rid of the ship and at one point tried to deny it even existed! Conspiracy? Yes. The ship was sold to Turkey and given a new name. Rumors of attempts to bend light around the ship as camouflage are false. The ship teleported and phased in and out of our reality in transition. Although seemingly impossible, the concept and experiments continue in the black world. The operation of the LHC also creates the possibility of a dangerous 4th weapon- a bomb that could create an artificial quantum singularity- mini black hole! It's probably best not to think about any of these things!