The sea

In his satirical Devil’s Dictionary, Ambrose Bierce defined the ocean as ‘a body of water occupying about two thirds of a world made for man – who has no gills’. Bierce may have been right to poke fun at human arrogance, but he underestimated the importance of the seas. Averaging almost 3,700 metres (12,000ft) deep, the ocean constitutes around nine tenths of the habitable space on our planet. It plays a commensurate role in the Earth system, not least as an engine – a ‘blue machine’ in the phrase that also titles an excellent book by the physicist and science presenter Helen Czerski – that moves heat around the planet.

I recently learned to dive in the bay of Dakar. It was exciting. I’d started learning in a Leeds swimming pool and though I knew the ocean would feel different, I didn’t expect it to feel comfortable. It shouldn’t. It is not my element, and humans have long since left it to the rest of the ocean’s creatures. I also didn’t think the ocean would sound like my neck when I roll it during yoga: that same crackle. With their remarkable sonar, dolphins can even tell when a human is pregnant That the ocean is not quiet is one of the most pleasing revelations of the past century (I mean