Every dive, your body soaks up inert gas — then has to let it back out. Decompression is just doing that release safely. No mystical timer counting down: it's physics you can actually reason about, and this guide walks the foundations in six short steps.
Under pressure, your tissues dissolve more nitrogen — that's Henry's law. Descend and gas flows into the tissues; ascend and the gradient reverses and it comes back out. The whole game is releasing it in a controlled way, without letting the pressure difference get out of hand.
Models like Bühlmann ZH-L16 split you into 16 theoretical tissue compartments. They aren't real organs — they're just maths tracking gas loading at 16 different speeds, from fast (blood, brain) to slow (fat, bone). Together they approximate how a real body loads and unloads.
Each compartment has a half-time — from about 5 minutes to 635. Fast tissues fill and empty quickly, driving short deep dives; slow tissues lag behind and drive long dives and repetitive schedules. Helium moves faster still, which is why trimix dives need their own compartment set.
On ascent, the gas pressure inside a tissue can exceed the surrounding pressure — that's supersaturation. A little is fine, and actually needed to off-gas at all. Too much, too fast → bubbles → DCS. Deco is the art of staying in the useful zone without crossing into the dangerous one.
The M-value is the maximum supersaturation a compartment tolerates at a given depth before symptoms appear. Fast tissues take more; slow tissues take less. Stay under every compartment's M-value line at every depth and you stay safe — that's what a deco algorithm is quietly checking every second.
Whichever compartment is closest to its M-value becomes the leader — it sets a ceiling, the shallowest depth you're allowed to be at right now. Stop below it, off-gas, and as the leader unloads the ceiling lifts. Follow the ceiling up and it walks you home to the surface.