Hot enough to boil oceans and vaporize rock. The highest terrestrial temperatures occurred more than four billion years ago, when a Mars-size proto-planet smashed into the Earth. (The debris from this collision formed our moon.) Within a millennium, the surface air temperature had dropped from a high of about 3,700°F down to 3,000°. Then the planet went into a period of slower cooling that lasted a few tens of millions of years. As the atmosphere thickened with heat-trapping water clouds and carbon dioxide and a shell of solid rock formed around the Earth's core, conditions stabilized at 440°.