The Hadean is the earliest eon in Earth's geologic history, beginning with the planet's formation. It occurred between approximately 4.6 billion and 4.0 billion years ago.
Over the course of the Hadean, Earth coalesced from a cloud of dust and gas, gradually taking shape while developing an atmosphere, oceans, and a moon. Very few rocks survive from this time, but the oldest known minerals, tiny zircon crystals, suggest that liquid water may have existed surprisingly early — even though Earth's surface was initially molten and marked by intense volcanic activity and bombardment by asteroids. This infernal environment inspired the eon's name, which refers to Hades, the Greek god of the underworld.