Reality is often very different from what it seems. The Earth appears to be flat but is in fact spherical. The sun seems to revolve in the sky when it is really we who are spinning. Neither is time what it seems to be. Let’s begin with a simple fact: time passes faster in the mountains than it does at sea level. The difference is small but can be measured with precision timepieces that can be bought today for a few thousand pounds. This slowing down can be detected between levels just a few centimetres apart: a clock placed on the floor runs a little more slowly than one on a table. It is not just the clocks that slow down: lower down, all processes are slower. Two friends separate, with one of them living in the plains and the other going to live in the mountains. They meet up again years later: the one who has stayed down has lived less, aged less, the mechanism of his cuckoo clock has oscillated fewer times. He has had less time to do things, his plants have grown less, his thoughts have had less time to unfold ... Lower down, there is simply less time than at altitude.