An airliner—especially one equipped with jet engines—must fly at very high altitudes, typically between 9,000 and 14,000 meters (approx. 30,000 to 45,000 feet), to minimize fuel consumption and cover long distances efficiently.
At these altitudes, the atmosphere is much thinner than at ground level, and it would be impossible to breathe. This is why airplane cabins are pressurized.
At these altitudes, the atmosphere is much thinner than at ground level, and it would be impossible to breathe. This is why airplane cabins are pressurized. They are designed to maintain an "artificial" atmosphere with the correct temperature and pressure to allow everyone on board to breathe normally.
The pressurization system usually consists of two or more automatic systems that "pump" air into the cabin, regulating it based on the aircraft's altitude.
Ideally, we would maintain the same conditions found on the ground throughout the entire flight (which would also prevent that "ear popping" sensation during climb and descent!). However, doing so would create a pressure difference between the inside and outside of the plane that is too great for the aircraft structure to withstand.
A compromise is reached: the "cabin altitude", as it is called, rises and falls following the flight profile of the aircraft, but with a much smaller range. At cruise altitude, the cabin pressure stabilizes at a value corresponding to an altitude of about 2,500–3,000 meters (8,000–10,000 feet)... just like going skiing in the Alps!
What happens if the pressurization system fails?
In such an event, the pilots would immediately begin a maneuver called an "emergency descent". This brings the plane down as quickly as possible to an altitude where passengers can breathe normally (usually around 3,000 meters or 10,000 feet).
For the duration of the descent, oxygen masks will automatically drop from the cabin ceiling and dangle in front of the passengers. All you need to do is grab one, pull it toward you, and put it on until you are told it is safe to remove it (see this link).
Mountains and "Escape Routes"
There are, however, parts of the world where it is impossible to descend immediately to a safe altitude due to high mountain ranges. Nothing is left to chance: for these specific geographical areas, pilots use "escape routes". These are specially designed paths that allow the pilots to navigate away from the mountains and reach a safe altitude before the oxygen supply for the masks runs out.
Every time we fly over such terrain, the pilots review the relevant escape route together so they are fully prepared to act immediately in the event of a depressurization.
DontWorryFlyHappy!