If I think about the concept of sea level hard enough, I experience disorientation, almost motion sickness, as awareness grows that I’m not living on solid ground but on a sinking chunk of planetary crust, on the surface of a not-really-spherical spinning globe, at the mercy of the nearest star, two icecaps and a capricious moon that sloshes the oceans to and fro like a child rocking in an over-filled bath.