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In between Adelaide and Perth lies the Nullarbor, a vast, dry plain. It’s famously flat; the railway line goes dead straight for almost 500 kilometres before it needs to turn. But hidden in the saltbush, scientists saw a strangely circular hill. So where did this circle come from?

The central dome is surrounded by a ring. And although it looks obvious in the image, this landform is harder to find in real life. The ring is less than 2 kilometres across, and the dome only reaches about 10 metres high. The international team of scientists, from Slovenia and Curtin University in Western Australia, spotted the striking circle in satellite scans!

There are plenty of big circles in nature. Volcanoes make them, and so do asteroid impacts. They can even be the remains of a collapsed cave. It’s hard to tell the difference between them just from a satellite map, so researchers went out into the field to look, and to collect rock samples.

Even before they set out, it was clear that the hill wasn’t a collapsed cave – the dome in the middle was too big. Back in the lab, the researchers didn’t find any evidence of volcanoes or asteroids. Instead, the rock looked like it came from an ancient reef!

Reefs such as the Great Barrier Reef have lots of ring shapes, including coral atolls. You’ll also find circular reefs built by seaweeds known as Halimeda. Based on the landform’s shape as well as evidence from tiny organisms found in the rock samples, there’s a good chance that this Nullarbor hill was once made from seaweed.

One mystery remains. How did this seaweed survive on a dry, desolate plain? Turns out, the Nullarbor wasn’t always dry. Fourteen million years ago, it was a shallow sea, perfect for growing reefs!