What is the habitable surface of the world? Actually, that is the wrong question. The right question is “What is the habitable volume of the world?”. It is easy to think that the ratio of marine habitat to land habitat is about 2:1—that is what we see when we look at the globe. But, this ignores the fact that, to a first approximation, the oceans are habitable in three dimensions, whereas the surface of the earth is only habitable in two. This makes the habitable volume of the seas vastly larger than our surface-biased eyes first intuit.