Air showers are cascades of secondary particles induced in the atmosphere by high energy cosmic rays. What you see here is a visualisation of realistic simulations of these showers. Of course, not all of the particles in a shower are displayed, there are far too many! The fraction displayed here is about 1e-6, sampled with a thinning algorithm. At a distance beyond some 100 meters from the shower axis, thinning is not applied any more. This way you can see decays or electron-positron creation.
Each square on the ground is one square kilometer.