Freediving challenges the physiological boundaries of the human body. Freedivers may stay underwater for up to 10 minutes, resisting the natural urge to release carbon dioxide and breathe in oxygen. The whole process goes against all human instinct which might be why some find freediving totally addictive. It becomes an immense mental challenge as well as a physical one.

It is more natural to hold the breath under water than above water. Once the face is immersed in water, the 'mammalian dive reflex' kicks in. The metabolism slows as does the heart rate, breathing is inhibited and blood starts to concentrate round the heart and lungs. The body begins to store oxygen rather than use it. The fingers and toes become numb. Because carbon dioxide still wants to escape, the body suffers contractions which can be painful. The abdominal muscles and chest severely contract and then relax.

A 'lung squeeze' can be dangerous as the lungs start to fill with blood. At this point the diver often feels so at peace and relaxed that they have trouble focussing on the procedures they should be following. The term 'narcosis' is used to describe this state of altered consciousness which occurs when diving at depth. The word comes from the Greek 'narke', a temporary decline of loss of movement and senses. Narcosis is similar to a state induced by nitrous oxide inhalation or alcoholic intoxication. While it can occur during shallow dives, it is more likely to occur at depths greater than 30 metres. Luckily, this 'altered state' is reversible.

While holding your breath beyond a certain period is not natural, it can become positively dangerous when practised deep in the ocean. The deeper the body is submerged, the greater the water pressure on the body. The rib cage and chest are pushed in; the stomach is pushed up into the chest space. This drastically reduces available lung volume. Density is generally measured as 1000kg per cubic metre. At a depth of 60 metres, the lungs become crushed to about one seventh of their normal size. Freedivers have died from getting their calculations wrong. In 2002, French diver Audrey Mestre died when she ran out of oxygen too far from the surface.

Free-DivingCredit: Wikimedia

Those who take up the sport often find it addictive and many now compete in freedive competitions held at various sites around the world. There are currently eight disciplines which may be contested. Three of these are performed in a swimming pool and five in open water.

Disciplines performed in a swimming pool
Dynamic Without Fins (DNF) involves travelling in a horizontal position underwater without propulsion aids of any description. The aim is to cover the greatest possible distance.
Record: Male: 218 metres    Female: 160 metres

Dynamic With Fins (DYN) is similar with the difference that fins, monofins and swimming movements are allowed.
Record: Male: 265 metres     Female: 225 metres

Static Apnea (STA) can be done in a pool or in open water. The freediver holds his breath underwater for as long as possible.
Record: Male: 11 minutes 35 seconds     Female: 8 minutes 23 seconds

Disciplines performed in open water
Free Immersion (FIM) involves pulling oneself down a cable to a set mark then ascending.
Record: Male: 121 metres    Female: 85 metres

Constant Weight (CWT) involves diving as deep as possible wearing flippers or a monofin.
Record: Male: 124 metres    Female: 100 metres

Constant Weight Without Fins (CNF) involves breast-stroking to a set mark without fins then returning to the surface.
Record: Male: 101    Female: 62

Variable Weight (VWT): Descent is accomplished with the help of a ballast weight and ascent by using the arms and/or legs.
Record: Male: 142 metres     Female: 126 metres

No Limit (NLT): Descent is with the help of a ballast weight and ascent by any method preferred by the diver.
Record: Male: 214 metres     Female: 160 metres

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The history of freediving stretches back to as far as 3000BC when Greek divers would dive for sponges. Its modern beginnings are generally recognised as being 1911 when a Greek from Karpathos retrieved the anchor of an Italian warship from the ocean floor. The story was later made into the film 'The Big Blue'.

One of the most popular destinations for freedivers is Dahab, a small town situated on the south-east coast of the Sinai Peninsula in Egypt. Dahab's Blue Hole acts as a magnet, drawing freedivers from all corners of the globe. Competitions are also held in New Zealand, Germany, Kiev, Italy, Greece and Cyprus.

The world-governing body for the sport is AIDA International which regulates the sport and arranges competitions.

Part of the fascination for freedivers is the immense feeling of blissful freedom and peace when sharing the underwater depths with fish, turtles and dolphins.

The world championships will be held in Greece from 15th to 25th of September.