The final course of the freediving certification is 40 meters deep. The reason is that human lungs contract and expand for breathing, and the volume of the lungs is reduced by water pressure as they enter the water. At a depth of 10 meters, the air is reduced to half its volume at ground level, at 20 meters it is reduced to one-third, at 30 meters it is reduced to one-fourth, and at 40 meters it is reduced to one-fifth. In order to breathe normally, 20% of the total lung size in men and 25% in women should be maintained to allow normal breathing. Therefore, a depth of 30 to 40 meters is the normal depth at which humans can dive normally. 

However, in normal freediving, diving to a depth of 50 or 90 meters is possible because of the mammalian diving reflex. When you hold your breath underwater, carbon dioxide increases in the body, blood moves to essential organs such as the heart and brain, and blood supply to peripheral parts decreases, resulting in numbness in the fingertips or toes and a decrease in heart rate, resulting in bradycardia. Usually, when diving, there are a lot of nerves in the lower part of the eye, so when cold water hits the face, the MDR reaction occurs, enabling breathing even at great depths. So even in the case of real dolphins, it is possible to dive in deeper water for a longer time than humans, but the depth in which they usually live does not exceed 40 meters. 

Another factor is buoyancy. At 15 to 20 meters, it is neutrally buoyant, meaning it neither floats nor sinks, as if in zero gravity. At greater depths, the body sinks due to gravity, which is called negative buoyancy, and the speed gradually increases, which is called free fall in diving. Finally, depending on how we control the brain, which uses the most oxygen in our body, we can dive while holding our breath for a long time. Due to human limitations and various external factors, the final gateway to freediving certification has become 40 meters. At a depth greater than that, various problems can occur at any time, so you will always need to be careful.

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