The coloured earth formed from cooled lava from ancient volcanoes. Over millions of years, the rock broke down into soil, and different minerals in the rock turned into different colours. The exact shade depends on which minerals are present and how much iron, aluminium or magnesium was in the original lava.
The amazing thing is that even when someone mixes the different coloured sands together, they slowly separate back into their own colours again. Scientists think this happens because the tiny particles of each colour are slightly different sizes, so they naturally settle apart from each other.
The dunes are quite small — only about 75 metres across — but they are stunning. People travel from all over the island and from many other countries just to see them. Nearby is the Chamarel waterfall, one of the tallest waterfalls in Mauritius, dropping into a lush green valley.