Sea level—perpetual flux. There is a micromillimetre on the surface of the ocean that moves between sea and sky and is simultaneously both and neither. Every known life-form exists in relation to this layer. Above it, the world of land, air, sunlight, and lungs. Below it, the world of water, depth, and pressure. The deeper you go, the darker, the more hostile, the less familiar, the less measured, the less known.