The body of an octopus is remarkable enough. The common octopus, O. vulgaris, has eight armlike appendages, three hearts pumping blue blood, an ink-based defense mechanism, and highly developed jet propulsion. An octopus can change size, shape, texture, and color at will, and all at the same time if necessary.

At three hundred feet, we are profoundly changed. The pressure at these depths is nine times that of the surface. The organs collapse. The heart beats at a quarter of its normal rate, slower than the rate of a person in a coma. Senses disappear. The brain enters a dream state. At six hundred feet down, the ocean’s pressure—some eighteen times that