Where did the oceans really come from?
Most scientists think they came from water-rich asteroids and comets raining down on the planet in its youth. Just after the Earth formed, it was very hot and dry. Prevailing theory suggests that millions of water-rich comets and asteroids bombarded our planet around 3.8 billion years ago, neatly explaining why oceans later appeared.
Also, the ratio of deuterium—or “heavy hydrogen” because it contains a neutron in addition to a proton—to hydrogen in our sea water matches the value found in water-rich asteroids, suggesting a common origin.