THE PANIC CIRCLE
When a diver encounters some stress underwater, real or imagined, his/her breathing and heart rate accelerates. Since the density of the air inhaled by the diver is higher at depth, the efficiency of the body's gas exchange decreases and blood levels of carbon dioxide increase. The increased carbon dioxide drives the body to breathe faster, in turn, further raising the levels of carbon dioxide and thus also increasing the diver's breathing rate.
As this cycle progresses, the diver soon becomes short of breath and feels "air starved." This creates additional stress and drives the diver towards a panic episode. In diving, panic is often described as "the complete and total loss of rational control that results when stimuli or conditions affecting a diver exceed his ability to respond reasonably".
Most of usually operate or function well within a zone of comfort. For example, I am comfortable operating a car, based on my experience and the driving conditions up to around 120 / 130 kph. The faster I drive, the farther away from my comfort zone I get. Alternatively, if speed remains constant, but the external conditions change dramatically, such as heavy rain - I will also get farther away from my comfort zone. When you leave the comfort zone, nearly any stimulus can initiate the panic circle, risking a total loss of control. Experienced divers and diving educators can generally recognise the potential causes of the panic circle and take steps to keep themselves and others well within the centre of the comfort zone by addressing problems before the dive or even underwater. Actions, like repairing a malfunctioning piece of equipment or merely terminating a dive, can completely short-circuit an emerging incident.