How old is the water on Earth?

Old enough that “the dinosaurs drank it” is the easy version of the question. The dinosaurs are 66 million years back. The water is billions — and where it came from is still being argued about by people whose job it is to know.

Start with what is not in doubt

Water is not being made and it is not being used up. It changes state — ice, liquid, vapour — and it moves, but the amount stays essentially the same. That is why this site can say the water you drink is the water that fell as rain on a dinosaur: not because it is a nice thought, but because there is nowhere else for it to have gone.

Run that backwards far enough and you stop asking how old the water is and start asking how it got here at all.

🔴 And here nobody can give you a straight answer

“How Earth ultimately became rich in liquid water has remained a source of debate for scientists.”

NASA Science, “NASA-Led Team Links Comet Water to Earth’s Oceans”, 3 December 2024. Credit: NASA.

That sentence is the most useful one on this page. A question can be old, important, worked on by thousands of clever people — and still open. Being told that is more honest than being handed a tidy story, and it is much more interesting, because it means the answer is still out there to be found.

Two of the ideas, in NASA’s words

It came out of the ground

“Research has shown that some of Earth’s water originated through vapor vented from volcanoes; that vapor condensed and rained down on the oceans.”

It arrived from space

“Researchers have found that water on Comet 67P/Churyumov–Gerasimenko has a similar molecular signature to the water in Earth’s oceans… this finding reopens the case that Jupiter-family comets like 67P could have helped deliver water to Earth.”

Notice the word reopens. The comet idea had been argued down, and then a measurement brought it back. That is not scientists being indecisive; that is what it looks like when evidence is actually being weighed.

Notice also that the two ideas are not enemies. Some of the water can have come out of volcanoes and some of it can have arrived on something that hit us. NASA says “some of Earth’s water”, not all of it.

So can water be billions of years old?

Yes — and whichever of those stories turns out to be right, it is billions either way. Water vented from a young Earth’s volcanoes is about as old as the planet. Water delivered by a comet was made before the comet arrived, which is older still.

What this page will not do is put a single number on it, because the sources it holds do not put a single number on it. “Billions” is the honest precision.

Next to that, the dinosaur question is almost cosy — and it is the one with a proper answer: is the water we drink dinosaur water? The volcano half of the story has its own page too: how are hot springs formed?

Where this comes from

The quotations: NASA Science, “NASA-Led Team Links Comet Water to Earth’s Oceans”, published 3 December 2024, read 1 September 2026. Credit: NASA. NASA content used factually, without implying NASA endorses this site. Every quotation above is verbatim; the sentences around them are our explanation, not extra facts.
How sure is this?: Less sure than anything else on this site, and deliberately so. This is a reconstruction of events four and a half billion years ago from chemical traces, and the source itself calls it a debate. If you read a page anywhere that tells you the answer plainly, check what it is citing.