Civilisation sprang from dirt. The thin layer of topsoil, formed on parts of the earth's surface over thousands or hundreds of thousands of years, enabled crops to be cultivated and gave early farmers a reliable food source. On average, that layer is only three feet deep.

But now that dirt is in danger. "The world's cropland is losing topsoil through erosion faster than new soil is forming, thereby reducing the land's inherent productivity," warns Lester Brown, founder of the Earth Policy Institute, in his book