In 1832, while HMS Beagle was anchored off San Salvador, Brazil, Charles Darwin went ashore to explore. There he saw rocks that glittered, as if burnished, in the sun. Darwin hypothesised that the shining was caused by a thin coating of metallic oxides but could not account for how it had been made. The strange phenomenon has since been observed in many places around the world, especially in deserts, and is now called desert varnish. But its origin – whether the result of complex but non-living chemical reactions or a residue left by unknown living forms – remains utterly mysterious.