Today, more than 70% of Earth is covered in liquid water. But long before the sea became a familiar feature of our planet’s surface, the water that now fills our oceans, lakes, and streams was swirling through the atmosphere and trapped in the molten rock beneath the surface.

Water is made of hydrogen and oxygen, and scientists think that both elements were present in some amount as Earth formed. It’s also possible that solar winds pushed additional H and O towards our planet and likely that even more arrived in rock form, as meteorites containing these elements crashed into Earth.

The early Earth churned with the energy from these collisions, and immense heat radiated into space. The early atmosphere was born as the Earth cooled, when gases from the hot interior bubbled up and escaped through vents in the surface like rifts and volcanoes. This atmosphere contained large amounts of water vapor alongside other gases like carbon dioxide and methane.

Inside the first rainclouds, raindrops formed the same way they do today. Water vapor began clinging to the tiny specks of dust floating in the atmosphere to form droplets, and when these droplets collided with each other, they combined into raindrops that were heavy enough to fall to the surface.

The problem was that Earth was still much too hot to accommodate liquid water, so these droplets just evaporated and floated back into the sky.

WATCH: ANCIENT EARTH FULL SERIES

Eventually, Earth had released enough of its heat into space for liquid water to remain on the surface. Massive rain storms flooded the young planet for millions of years, filling the cracks and basins in the rocky surface and creating the first streams and oceans. With that accomplished, the planet was ready to host life as we know it.