The most popular scientific theory says Earth's water comes from asteroids and comets. The asteroid, filled with ice, entered Earth, which was still dry and rocky at that time, about 4.5 billion years ago. Then, some say that Earth's water also comes from comets which store water.
However, a recent study of comet Hale-Bopp, Earth's water does not come from comets. Comets have a water component as their constituent material. This water is different from the water on Earth. This water contains the hydrogen isotope, namely deuterium, and is more often referred to as "heavy water".