Step 1: Earth was born as a fiery world

About 4.54 billion years ago, Earth formed from dust and rock orbiting the young Sun.

The early Earth was incredibly hot because:

• Countless asteroid collisions released enormous amounts of energy.
• Radioactive elements heated the interior.
• The planet was still compressing under its own gravity.

Much of the surface was covered by a global ocean of molten rock, often called a magma ocean.

There could be no liquid water—it was simply too hot.

Step 2: Water was trapped inside the planet

Even though the surface was molten, many of the rocks that formed Earth contained small amounts of hydrogen and oxygen locked within their minerals.

As the interior melted, volcanoes released enormous quantities of gases, including:

• Water vapor
• Carbon dioxide
• Nitrogen
• Sulfur compounds

This process is called volcanic outgassing.

At first, the atmosphere may have contained more water vapor than today's oceans contain as liquid water.

Step 3: It finally became cool enough to rain

As Earth gradually cooled over tens of millions of years, the atmosphere reached a point where water vapor could condense.

Then something extraordinary happened.

It likely rained for thousands to perhaps millions of years (not continuously every minute, but over an extended period as the planet cooled).

Instead of evaporating again, the rain collected in low-lying basins.

The first oceans were born.

Step 4: Space added more water

While Earth's own volcanoes supplied much of the water, another source was arriving from space.

Small icy and water-rich asteroids frequently struck the young Earth.

Scientists now think these water-rich asteroids contributed significantly to Earth's oceans. Comets probably contributed some water too, but most evidence suggests they were a smaller source than asteroids.

Researchers compare different forms of hydrogen in Earth's oceans and in meteorites. Many meteorites have water with a chemical "fingerprint" that closely matches Earth's oceans, supporting this idea.

Step 5: The oceans kept changing

Earth's oceans haven't stayed the same.

Water is constantly moving through the planet in a cycle:

• Volcanoes release water from the mantle.
• Oceanic crust carries water back into the mantle where tectonic plates sink.
• Some water remains locked inside minerals deep within the Earth.

Scientists estimate there may still be one to several ocean's worth of water stored within Earth's mantle, bound inside minerals rather than existing as liquid.

How much water is in the oceans?

Today, Earth's oceans contain about:

• 1.33 billion cubic kilometers (320 million cubic miles) of water.
• They cover about 71% of Earth's surface.
• Yet they make up only about 0.02% of Earth's total mass.

If all the oceans were spread evenly across the globe, Earth would be covered by a layer of water about 2.7 kilometers (1.7 miles) deep.

Do scientists know exactly where all the water came from?

Not yet. The leading explanation is:

• Most of the water came from Earth's interior through volcanic outgassing.
• A substantial amount was delivered by water-rich asteroids during the planet's early history.
• Comets likely made a smaller contribution.

This combined model best matches the geological and chemical evidence we have today.

In a sense, every drop of seawater has an ancient history: some of its molecules were trapped inside the rocks that built our planet over 4.5 billion years ago, while others may have traveled through space aboard primitive asteroids before becoming part of Earth's oceans.