And humanity has grown in activity. An impressive example is deforestation. Once upon a time, Earth's surface to a large extent was covered by forrests, dense forests in many areas that it was difficult for humans to actually dwell in them. A hypothethese states that, for example, four thousand years ago, the British Isles were covered with a forrest so dense that humans moved along the coasts to the North but did not explore the interior. The map shows the extent of the original forrests.