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The ground beneath our feet seems to be the most solid and unchanging things we know. It forms foundations for our cities and an environment in which we can live. Yet in reality Planet Earth is spinning on its axis and hurtling through hostile space, as it orbits the nuclear furnace of our Sun. It is an active, dynamic, living planet.
The rock-solid surface is not as solid as it appears. It is cracked like giant slabs of crazy paving. Earthquakes shake our cities and volcanoes erupt, giving clues to the fiery movements beneath the ground. From above, the planet is buffeted by radiation and particles streaming as it through space. But in between is an atmosphere, and oceans with liquid water, and temperatures which are just right for life.
To an alien spacecraft travelling past the solar system, Planet Earth would immediately stand out as something special. The composition of the atmosphere, with free oxygen and traces of gases like methane can only be sustained by life. The aliens might detect characteristic colours of chlorophyll, the pigment used by plants on land and algae in the sea to trap sunlight. If our aliens were not careful they would pick up the radio cacophony of our broadcasts, revealing that life here is at least moderately intelligent.