Coccolithophore

Chalk formations such as the white cliffs of Dover are largely composed of the bodies of countless billions of coccolithophores that bloomed, died and then sank to the seabed each season for millions of years.

It was filled with a kind of golden-brown planktonic algae known as coccolithophores, whose skeletons would form the chalks that underlie parts of Britain, Belgium and France today.