This chapter discusses life under high atmospheric pressure, where the specific gravity of water is much higher than that of air. The physiological problems associated with undersea exploration may be divided into those faced under the sea, and those faced during return to the surface. Enormous volumes of air have to be supplied to maintain the normal tidal volume. Free radicals oxidize the polyunsaturated fatty acids present in the membranes of cells and organelles and also oxidise some intracellular enzymes. Oxygen toxicity manifests as disorientation and dizziness, or even convulsions and coma. Aerated drinks contain carbon dioxide dissolved under high pressure. Nitrogen has a depressant action on the nervous system. Carbon dioxide build-up leads to respiratory acidosis and narcosis. At higher pressure, nitrogen dissolves gradually into body fluids, and even more easily into fats. As a result of breathing nitrogen, nitrogen dissolves in body fluids and fats.