The first thing you notice underwater is the silence.

The noise of the boat fades away. Conversations are replaced by simple hand signals. Your breathing becomes the loudest sound in the world. Shafts of sunlight ripple across the seabed, fish glide effortlessly past, and for a moment you’re completely absorbed by the world around you.

Before your ears begin to ache or your breathing starts to feel different, your body has already begun adapting to an environment it was never designed to inhabit. The ocean feels calm because your body is working so hard to keep it that way.

Meanwhile, someone else in the group isn’t having such a great time.

There’s always one. One diver whose ears refuse to equalise. Another whose mask continually fills. They tell you not to fly after a dive, but nobody really understands why. None of these experiences are coincidence. They’re all the body’s response to a single invisible force.

An Invisible Force

The ocean doesn’t know whether you’re taking your first breath underwater or your thousandth dive. It doesn’t care how experienced you are, how fit you are, or how far you’ve travelled to get there.

Every dive is a conversation between the human body and the laws of physics. The ocean never changes. Our bodies simply learn how to adapt.

Pressure changes remarkably quickly underwater. Water is around 800 times denser than air, so pressure increases far more rapidly underwater than it does at altitude. Every 10 metres of seawater adds another atmosphere of pressure. By just 10 metres, the pressure around your body has already doubled. By 30 metres, it’s four times higher than it was on the boat only a few minutes earlier.

The ocean doesn’t compromise. Your body either adapts to the changing pressure, or
it’s time to head back to the surface.

The First Few Metres

Ironically, the first few metres of a dive are often the hardest.

Not because they’re the deepest, but because this is where the greatest proportional
change in pressure occurs – which is why beginners often struggle most near the surface.

Most of your body barely notices. That’s because muscles, organs and blood are made
largely of water, which compresses very little.

It’s the gas inside us that responds to pressure.

Your ears are usually the first to tell you.

Behind each eardrum sits a small pocket of air called the middle ear. As you descend, increasing pressure compresses that trapped air, pulling the eardrum inwards. The Eustachian tube, which connects the middle ear to the back of the nose, is normally closed. It opens briefly when you swallow, yawn or perform an equalisation manoeuvre, allowing fresh air into the middle ear and restoring equal pressure on both sides of the eardrum. Unless this happens, ears quickly become very painful.

Experienced divers know not to wait until their ears hurt. They equalise early, often before they feel anything at all. If equalising becomes difficult, ascending a metre or two usually restores enough balance to try again.

More Than Just Your Ears

The ears aren’t the only air-filled spaces responding to pressure.

Your sinuses are connected to the nose through tiny drainage channels that allow air to move freely. If those passages become blocked by a cold, allergies or inflammation, pressure can no longer equalise normally.

During descent, the shrinking pocket of air creates a partial vacuum that pulls painfully on the delicate lining of the sinus. During ascent, the opposite happens. Expanding air
may become trapped behind swollen tissues, creating pressure from within.

If you’ve ever tried diving with a cold, you’ll probably have promised yourself never to do
it again.

Even your diving mask contains air.

As pressure increases, the volume of air inside the mask becomes smaller, creating suction against the face. A gentle breath out through the nose replaces the compressed air and equalises the pressure inside the mask. Forget to do so, and you may surface with a mask squeeze – bruising or bursting of tiny blood vessels around the eyes. Harmless, but memorable.

Even your teeth can join the conversation.

Tiny pockets of air trapped beneath an ageing filling, inside a cracked tooth or within an
untreated cavity can expand and contract as pressure changes. The result is a sudden sharp pain known as tooth squeeze. For some divers, it’s the first indication that a filling
has begun to fail.

By now, a pattern is probably becoming clear.

Wherever gas is trapped inside the body, pressure matters.

Why Breathing Feels Different

Breathing doesn’t feel different because there is less air.

The difference is the density of the air.

Your regulator delivers air at exactly the same pressure as the surrounding water, allowing your lungs to inflate normally wherever you are.

At 30 metres, every breath contains roughly four times as many gas molecules as it did at the surface. That denser gas creates greater resistance as it moves through your airways, meaning your respiratory muscles have to work harder with every breath.

Cold water, anxiety and swimming against a current all add to that workload.

That’s one reason air consumption increases with depth, even when you’re perfectly relaxed.

The calmer the diver, the easier breathing usually feels.

Never Hold Your Breath

If there’s one rule every diver remembers, it’s this: Never hold your breath.

As you ascend, the pressure around you falls and the air inside your lungs naturally expands.

If you’re breathing normally, that expanding air simply leaves the lungs with each breath.

Hold your breath, however, and the expanding gas has nowhere to go. The lungs are delicate structures made up of millions of tiny air sacs. Trapping expanding gas inside them can overstretch those tissues, allowing air to escape outside the lungs, into the chest, beneath the skin or into the bloodstream, where it can travel to other organs such as the brain or heart.

It’s one of the few serious diving injuries that’s almost entirely preventable.

Why Nitrogen Matters

Most divers spend their time thinking about oxygen.

Ironically, it’s nitrogen that tells the more interesting story.

Every breath you take contains around 79% nitrogen. At the surface, your body pays it very little attention.

Under increased pressure, more nitrogen passes from the lungs into the bloodstream before gradually dissolving into tissues throughout the body. The deeper and longer you dive, the more nitrogen accumulates.

You don’t feel it happening.

As you begin your ascent, the process quietly reverses. Nitrogen leaves the tissues, returns to the bloodstream and is carried back to the lungs, where it’s breathed out harmlessly.

Provided the ascent is slow enough…

When It All Goes Wrong

Problems arise when the body doesn’t have enough time to eliminate that nitrogen.

Instead of remaining dissolved, it comes out of solution as tiny nitrogen bubbles.

This is decompression sickness.

Many people imagine decompression sickness as a diver surfacing dramatically before immediately collapsing.

Reality is often far less obvious.

In fact, the earliest symptoms are often subtle enough to be dismissed as nothing more than tiredness after a long day in the water.

Symptoms may not appear for several hours, and no two people present in exactly the same way.

Some develop a deep aching pain in the shoulders, elbows, knees or hips. The joints themselves aren’t damaged. Instead, nitrogen bubbles irritate the tissues around them, producing pain that ranges from mild stiffness to severe discomfort.

Others notice itching or a marbled, blotchy rash spreading across the skin. Although it can look relatively harmless, it may be an early sign that bubbles are affecting the circulation.

The nervous system is particularly vulnerable.

Nitrogen dissolves especially readily into fatty tissue, and because the brain and spinal cord contain large amounts of fat, they are among the organs most at risk. Pins and needles, numbness, weakness, difficulty walking, problems with balance or changes in bladder function all require urgent assessment.

More severe cases can affect the lungs, causing chest pain, persistent coughing and breathlessness, or the brain, leading to confusion, visual disturbance or loss of consciousness.

One of the easiest symptoms to dismiss is overwhelming fatigue. Some divers simply describe feeling unusually exhausted after what seemed like an ordinary dive.

Decompression sickness can affect almost any organ because bubbles don’t simply block blood flow. They also trigger inflammation and damage the lining of blood vessels, which is why symptoms are so variable.

The sooner treatment begins, the better the outcome.

Helping Your Body Adapt

Most cases of decompression sickness occur after dives that appeared to go entirely to
plan.

Fortunately, modern diving is built around giving the body the time it needs to adapt.

Dive computers continuously calculate nitrogen loading, guiding divers on how long they can remain at depth and how slowly they should ascend. For longer or deeper dives, planned decompression stops provide additional time for dissolved nitrogen to leave the body safely before reaching the surface.

Even recreational divers are encouraged to make a short safety stop, typically around five metres for three minutes. Although not always mandatory, it provides extra time for nitrogen to continue leaving the tissues before surfacing.

Much of safe diving is simply giving your body the time and conditions it needs to do what it was already trying to do.

Hydration also matters.

When you’re dehydrated, blood becomes more concentrated and circulation may become slightly less efficient, potentially slowing the movement of nitrogen back to the lungs. Although dehydration alone doesn’t cause decompression sickness, maintaining good hydration is one simple way of reducing unnecessary physiological stress.

Alcohol creates problems for several reasons.

It contributes to dehydration, impairs judgement, slows reaction times and can make it more difficult to recognise the early symptoms of decompression sickness.

Another piece of advice every diver hears is to avoid flying soon after diving.

At first glance, it seems strange. After all, the dive is already over.

The explanation lies in pressure once again.

Although aircraft cabins are pressurised, they are not maintained at sea-level pressure.

Instead, cabin pressure is typically equivalent to an altitude of around 6,000–8,000 feet (1,800–2,400 metres). This small reduction in pressure is enough to encourage any remaining dissolved nitrogen to come out of solution more quickly, increasing the risk of
decompression sickness.

For that reason, most recreational diving organisations recommend waiting at least 12–
24 hours before flying, depending on the number and type of dives completed.

The dive may have finished, but your body is often still quietly adapting long after you’ve climbed back onto the boat.

The Ocean Doesn’t Change. We Do.

Nature rarely bends its rules for us. Mountains don’t. Jungles don’t. Neither does the ocean.

The sea isn’t trying to make diving difficult. It simply obeys the laws of physics. The more we understand those laws, the more comfortably and safely we can explore it.

The ocean doesn’t know whether you’re on your first dive or your thousandth. Pressure treats every diver exactly the same. Experience doesn’t change the laws of physics. It simply teaches you how to work with them.

Understanding those laws, respecting the natural world, and giving your body the time it needs to adapt are what allow us to safely explore some of the most extraordinary environments on Earth.

Information and other content provided in these blogs should not be construed as medical advice and should not be considered a substitute for professional medical expertise. If you have any medical concerns, you should consult with your health care provider.