The gleaming White Cliffs of Dover have dark seams of flint within them that were the target of some of Britain’s first mines—more than six thousand years ago.

The white cliffs are made of nearly pure chalk, the remains of phytoplankton, tiny floating algae.

These phytoplankton lived here in warm Cretaceous seas. When they died, their spherical bodies fell apart into the hard plates that covered them.

The microscopic plates, made of calcium carbonate, sank to the seafloor. There they formed a layer of white mud, at a rate of just half a millimeter per year.

But over 30 million years, that white sediment layer reached nearly two thousand feet thick. Its weight compacted it into the chalk we see today.

The main consumers of these floating algae were radiolarians, tiny zooplankton whose bodies were made not of calcium but silica.

Once they died, and were compacted for millions of years, the silica in their remains formed layers of flint in the chalk.

Though flint is a form of quartz, it’s nearly as hard as a diamond. It can be chipped to have very sharp edges, which Stone Age humans used to make blades and hunting points.

It was so valuable that Neolithic tribes would dig mine shafts down through the soft chalk to reach a strip of flint, then bring it to the surface to work into tools.

These mine shafts, and the tool making sites around them, may be Britain’s earliest industry and one of the reasons that human population grew here.