Soon after the solar system formed out of a cosmic cloud of gas and dust more than 4.5 billion years ago, Earth was a dry, cratered ball barren of life. It had no ocean and no continents. As the eons rolled by, our planet was transformed into a water world teeming with myriad forms of life. How did such a dramatic transformation occur?

Scientists believe that Earth in its earliest years was a horribly hot and violent place. Asteroids, comets, and other chunks of space debris left over from the solar system's formation continually bombarded the young planet, releasing huge amounts of heat. The decay of radioactive elements inside the Earth also generated great quantities of heat. At the same time, frequent volcanic eruptions may have covered much of the planet's surface in red-hot flows of lava. According to scientists, the early Earth's surface was hot enough to turn any liquid water instantly into steam. Nonetheless, the planet somehow obtained enough water to eventually form a vast ocean.

Scientists have long sought to explain where this water came from. Some researchers have theorized that comets, which contain large amounts of water ice, delivered water to Earth during the millions of years that comets pummeled the young planet. Other investigators, however, dispute this view. They point out that in the 1980's and 1990's, scientists used radio telescopes to analyze the chemical makeup of three comets as they came close to Earth. These studies revealed that comets contain 2 to 20 times more “heavy water” (relative to “normal” water) than is found in the ocean. Normal water molecules are composed of one oxygen atom and two hydrogen atoms. Heavy-water molecules contain at least one atom of deuterium, an isotope (variant form) of hydrogen that has twice the mass of ordinary hydrogen. The higher proportion of deuterium in comet ice suggests that comets could not have been a major source of seawater.

Most scientists now believe that the ocean originated from water molecules in the cloud of gas and dust that gave rise to the solar system. According to the current theory, as Earth formed, these water molecules first became trapped in porous rock deep inside the hot planet, but they later boiled out as steam. For hundreds of millions of years, volcanoes ejected the water vapor along with other hot gases into the Earth's atmosphere. The volcanoes spewed out enough water vapor to wrap the entire planet in a dense blanket.

After the meteorite bombardments and volcanic eruptions subsided, Earth finally cooled enough to permit the water vapor to condense into clouds and fall as rain. An almost continuous rain, fed by evaporation and volcanic outgassing, may have drenched the planet for some 10 million years. As the deluge persisted, ponds formed in shallow depressions and then grew into lakes. Eventually, the lakes merged to form an ocean. Many researchers now believe that the first permanent ocean was likely in place sometime between 4.3 billion and 3.8 billion years ago.

The early sea may have covered the entire planet to an unknown depth as rain water collected everywhere on the nearly featureless landscape. Raised continents and deep ocean basins had not yet formed. These features would take shape after geological processes began altering the landscape.