Breathing underwater is applied physics.

At sea level you're already under one "atmosphere" of air: about 1.013 bar, which we'll call 1 bar. Water is roughly 850 times denser than air, so it piles on pressure fast. A column of seawater 10 m tall over one square metre holds 10 m³ × 1,025 kg/m³ = 10,250 kg. Its weight (× 9.81 m/s²) spread over that square metre is about 100,500 pascals. That's ≈ 1 bar.

So: every 10 m of seawater adds one more atmosphere. Your gauge shows depth, but your body responds to absolute pressure, which is air plus water.

Boyle's law: squeeze the gas, shrink the gas

For a fixed amount of gas at constant temperature, P₁ × V₁ = P₂ × V₂. Double the pressure and the volume halves. Your body is mostly liquid, and liquids barely compress. The gas spaces are what feel it: lungs, middle ears, sinuses, mask, BCD, the bubbles in your wetsuit.

The one rule that's pure physics: never hold your breath

Your regulator gives you air at whatever pressure surrounds you. Fill your lungs with 6 L at 10 m (2 bar), then swim up holding that breath. By the surface (1 bar) that same gas wants to take up 12 L. Lungs can't double in size. Small air sacs tear, and air can be forced into the chest cavity or bloodstream (pulmonary barotrauma, arterial gas embolism). Diving-medicine literature describes these injuries after breath-hold ascents of only a metre or two, so this isn't just a deep-dive risk. The fix is simple: breathe slowly and continuously, all the time, and on an emergency ascent keep exhaling ("ahhh").

Archimedes' principle: anything in water is pushed up by a force equal to the weight of the water it displaces. Displace more water than you weigh and you float. Displace less and you sink. Match exactly and you hover. That's "neutral buoyancy", the skill that separates calm divers from flailing ones.

Every litre you displace in seawater (density ≈ 1.025 kg/L) gives you about 1.025 kg of lift. In fresh water it's 1.000 kg. Your fine controls are all gas volumes: BCD air, wetsuit bubbles and your lungs.

The shrinking wetsuit

Neoprene is a foam full of tiny gas cells, and Boyle's law applies to them too. Say a full suit gives you about 4 kg of lift at the surface (it varies a lot with thickness and size). If it behaved like pure gas it would give ~2 kg at 10 m and ~1 kg at 30 m. Real neoprene doesn't quite compress that fully, but the trend is real: you get heavier as you go down. You make up for it with small puffs into the BCD on descent, then vent that air as it expands on ascent.

You get lighter as you breathe

Trim: the physics of looking effortless

Water is about 850× denser than air, so drag dominates everything. Drag grows with your frontal area and with the square of your speed, which means the power you need grows with the cube of speed. Swim twice as fast and you need roughly 8× the power. A diver swimming at a 45° angle pushes a much bigger cross-section through the water than one lying flat. "Trim" (horizontal, streamlined, gear tucked in) cuts drag, and that cuts effort and air use (Chapter 3). Slow and flat wins.

Your regulator delivers air at the surrounding pressure, or you couldn't inhale. So a lungful at 30 m (4 bar) holds four times as many air molecules as a lungful on the beach, and it drains the cylinder four times as fast.

Divers measure breathing as SAC (surface air consumption): litres per minute at surface pressure. A relaxed recreational diver might be around 15–25 L/min. Cold, stress, currents and poor trim push it up, sometimes a lot. We'll use 20 L/min.

Why keep a reserve? Do the ascent maths

From 30 m, rising at 9 m/min takes ~3 minutes at an average of about 2.5 bar: 20 × 2.5 × 3 ≈ 150 L. A 3-minute safety stop at 5 m (1.5 bar) costs 20 × 1.5 × 3 = 90 L. That's ~240 L, or 20 bar from a 12 L tank, for a calm diver. Now picture sharing air with an out-of-air buddy who's breathing hard: two people, maybe double the SAC. A 50-bar reserve stops looking conservative.

Air is about 79% nitrogen and 21% oxygen. Each gas behaves as if it were alone, with a partial pressure = fraction × absolute pressure (Dalton's law). Henry's law says the amount of gas that dissolves in a liquid is proportional to its partial pressure. Your blood and tissues are the liquid.

Loading up: tissues with half-times

At 30 m, you're breathing nitrogen at 3.16 bar while your tissues are still at ~0.79. Nitrogen flows in. It doesn't flow in all at once. Decompression models picture the body as a set of "compartments", from fast ones like blood to slow ones like joints and fat, each with a half-time. Common models span half-times from a few minutes to more than ten hours.

Coming up: why slow ascents and safety stops work

A modest supersaturation leaves your tissues harmlessly through the lungs. Too much, too fast, and the gas forms bubbles in tissue and blood. That's decompression sickness ("the bends"): joint pain, numbness, fatigue, and in serious cases paralysis. Recreational divers stay within no-decompression limits (NDLs), ascend slowly (agency maximums are 18 m/min, and many computers want ~9–10 m/min), and add a 3-minute safety stop at about 5 m.

*Published values of the PADI Recreational Dive Planner (air), for illustration. Computers and other tables differ. Note how fast the limit collapses with depth.

Nitrogen narcosis: the "martini effect"

At high partial pressure, nitrogen acts like a mild anaesthetic: slowed thinking, overconfidence, fixation, giggles or anxiety. Many divers start to notice it around 30 m, which is one reason the Advanced level stops there. It varies between people and from day to day. Two things make it manageable: it gets stronger the deeper you go, and it fades within moments when you go up a few metres. No hangover.

Oxygen: essential, then toxic

Breathing oxygen at a high partial pressure can trigger convulsions without warning, which underwater can mean drowning. Recreational training uses 1.4 bar as the working limit (1.6 as an absolute contingency). That sets a maximum operating depth (MOD) for each gas:

Behind each eardrum is the middle ear, a small pocket of air connected to the back of your throat by the narrow Eustachian tube. As you descend, water pushes the eardrum inward. The air behind it stays at surface pressure until you open the tube and let higher-pressure air in. That's equalising.

Remember the Boyle table: the change from 0 to 2 m (1 → 1.2 bar) squeezes an air space by about 17%. From 30 to 32 m it's only 5%. The shallows are where equalising matters most.

How to equalise: early, often, gently

• Start before you leave the surface, then equalise every half-metre to metre in the first few metres, and keep going all the way down.
• Valsalva: pinch your nose and blow gently against it. Never hard.
• Toynbee: pinch your nose and swallow. Frenzel: pinch your nose and push air up with your tongue like a piston. It takes a bit of practice but is very gentle.
• Wiggle your jaw. Descend feet-first, which helps air rise into the tubes.
• If it hurts, go up a metre and try again. Never push through pain, because a burst eardrum or inner-ear injury can end your diving.
• Don't dive congested. Decongestants can wear off mid-dive and trap air on the way up (a "reverse block").

The same physics applies to your sinuses (which equalise on their own through small openings, so congestion blocks them) and your mask: breathe out a little through your nose on descent, or the mask "squeezes" your face.

Why reds vanish first

Water molecules absorb light, and they absorb long wavelengths (red) far more strongly than short ones (blue). Light fades exponentially with distance (the Beer–Lambert law): I = I₀ × e^(−a·d), where a is the absorption coefficient for that colour. Using laboratory values for pure water:

Share of surface sunlight left travelling straight down through pure water (coefficients: Pope & Fry, 1997). Real seawater also has plankton and sediment, which absorb and scatter more, especially along India's west-coast mainland sites.

The mask is a magnifying glass

Light bends where it passes from water (refractive index ≈ 1.33) through your flat mask into air. Objects appear at about three-quarters of their real distance, so they look about a third bigger. That "2-metre" fish was probably 1.5 m, and the reef you're about to touch is further away than it looks.

Sound: louder, faster, directionless

Sound travels at about 1,500 m/s in seawater versus ~343 m/s in air, roughly 4.4× faster. Your brain locates sounds mostly from the tiny difference in when they reach each ear. Underwater that difference shrinks by the same factor, and sound also travels through your skull bones straight to both inner ears. Result: you hear a boat engine clearly but can't tell where it is. Treat engine noise as "look up, stay close to the line or reef, and surface carefully", ideally with a surface marker buoy.

Heat: why 28 °C water still chills you

Water conducts heat about 25× faster than air (≈0.6 vs ≈0.025 W/m·K) and holds vastly more heat per litre. Your body is at 37 °C, so even the warm 26–30 °C seas of the Andamans and Lakshadweep steadily pull heat out on a long dive or a third dive of the day. A wetsuit works by trapping a thin layer of water that your body warms. Cold also raises your air consumption (Chapter 3).