An illustrated ocean shore at a bright dawn, huge waves breaking on dark rock, volcanic islands steaming on the horizon under a sky streaked with pale light.

How has Earth’s water changed through time?

Less than you might think, and more. The water itself is old, and Earth has been recycling that enormous ancient supply for billions of years. What changes is everything else: its molecules are broken apart and rebuilt, and the share of the world under the sea goes down and comes back up.

AI reconstruction — not a photograph · Shutterstock AI/Shutterstock.com · Shutterstock Standard License

Four pages, one long story

How old is the water on Earth?Old enough that “the dinosaurs drank it” is the easy version, and where it came from is still being argued about.
Is the water we drink dinosaur water?Almost certainly, yes, in its atoms, even though no single molecule can be followed from a dinosaur to your glass.
What oceans existed during the dinosaurs?Two named oceans, shallow seas on top of the continents, and how much of the world was under water at each mapped moment.
Can volcanoes create water?Volcanoes give off water, lots of it, but giving off and creating are different things, and that gap is the whole page.

Real photographs: 300785485/Shutterstock.com · James St. John, CC BY 2.0, via Wikimedia Commons · 228988681/Shutterstock.com · Shutterstock Standard (free image collection, API) License · CC BY 2.0

Read them in this order

Start with how old the water is, since that is the question the other three grow out of. Then the dinosaur-water page shows what old means for the glass in your hand, and why it is the atoms that carry on rather than the molecules.

The oceans page is where the change shows: it lays every map this site holds in a row and counts the sea on each one. The volcano page asks the last question, whether any of the water is new, and refuses to pretend the answer is simple.

Where next

To see the maps themselves, period by period, go to What did Earth look like when dinosaurs lived? For the rivers that carry this water today, and how young two of them turn out to be, try How old are rivers?