A sealed suit fails alone. Under a deep water column, hydrostatic pressure bears on every air-filled cavity, while a surface-breathing animal's lungs and blood still need oxygen at a usable partial pressure, not dry skin and a zipper that closes. The body keeps accounts.
Breathing is the harder bargain. If supplied gas reaches the chest below ambient pressure, alveoli cannot inflate; if oxygen partial pressure rises too far, toxicity can follow, while inert gas enters blood and tissue under compression. No fabric can bargain. A regulator must match delivery pressure to depth, and the breathing mix must keep oxygen adequate without letting the dose become hazardous; carbon dioxide must also be removed, because its buildup drives hypercapnia even when oxygen is present. The margin shrinks fast. At greater pressure, denser gas raises the work of breathing, turning each inhalation into a mechanical demand rather than a simple reflex.
Ascents expose the hidden bill. As surrounding pressure falls, dissolved nitrogen can leave solution as bubbles, so decompression schedules, controlled ascent rates, and staged stops limit barotrauma and decompression sickness. That margin is narrow. Rapid expansion can injure ears, sinuses, and lungs, while bubbles in blood or tissue may obstruct circulation and inflame tissue. Pressure remembers everything. At depth, a suit is not a shelter so much as the outer wall of a life-support system, whose unseen work decides whether the return to air is orderly or violent. Water holds its silence, and the body must answer it.
