
Twenty thousand years ago the world was locked into a great ice age. Ice sheets two miles thick covered much of North America, Scandinavia and the British Isles.
Greenhouse gas concentrations were much lower, the world was 6°C colder, and because of all the water trapped in ice-sheets, the sea was at least 120 metres lower, exposing land that is submerged today. It would have been possible to walk from France to London via Doggerland or from Russia to Alaska through Beringia.