It is thought that comets may have seeded Earth's oceans. After the planets had largely formed, there was a period lasting several hundred million years, during which the young worlds continued to accrete debris, raining down from space. This was called the heavy bombardment phase. Much of this debris would have been in the form of comets, rich in water. In time, as these impacts began to subside and the Earth cooled, the water would have condensed out, or rained from the atmosphere, or both, forming what would eventually become our planet's oceans