All the air: All the air in the atmosphere (5140 trillion tonnes) gathered into a ball at sea-level density. The sphere is 2,000 km across. Air gets thinner the further you are from the surface of the Earth. Half the air in the atmosphere lies within 5.5 km of the ground.
Looking at the Earth can be an empowering experience: it can alter our sense of relationship with it and our role in protecting it. Earth Day is a celebration of the Earth but also of historic and continuing efforts to look after it. So this is a good moment to reflect on the importance of really engaging with the Earth.