Did you know that until very recently, more people had been to the moon than had dived beyond depths of six thousand metres? I think about this often – the inhospitableness of certain places. A footprint, once left on the surface of the moon, might in theory remain as it is almost indefinitely. Uneroded by atmosphere, by wind or by rain, any mark made up there could quite easily last for several centuries. The ocean is different, the ocean covers its tracks