1. The air

An invisible ocean, air surrounds the Earth in a gigantic envelope several hundred kilometers thick, yet it is one of the least studied environments. Perhaps simply because, of all the elements that nature offers us to live with on a daily basis, it's the only one we use without thinking. We think of drinking and eating, but never of breathing; air is neither palpable nor visible, and although it penetrates everywhere, we forget it exists.

However, we've forgotten so much about it that it's beginning to remind us of its existence, in a discreet but increasingly significant way. Without borders, climatic events, and especially winds, turn the atmosphere into a vast mixture that can carry dust and bacteria from the Sahara to the West Indies.