I recently saw this visualisation of the northern European airspace returning to use after being closed due to the volcanic ash from Eyjafjallajokull. I was struck at first by how oddly beautiful the lines of light zipping about were. Then I was struck by how terrifying the idea is of all that metal and all those people flinging across the sky. It also brought home the environmental impact of air traffic. Then I realised that this was just the area of northern Europe and glimpses of its neigbours. Add in the rest of the world and the scale of what is zipping about up there beggars belief.
Watch and be dazzled. Then maybe feel a bit scared.