ABSTRACT
Visitors from outer space would on approaching the planet Earth, make out the continents, then large islands and natural features such as the Great Barrier Reef off the north-east coast of Australia. On circling the Earth away from the sun, they would be drawn to the sparkling lights of cities and busy motorways. With the sun behind them, they might detect the Great Wall of China and almost certainly canals, railway lines and road networks. The extent of forest clearings due to the spread of new roads far from any cities (Fig. 1.1} might puzzle them.
