Air lock is a device that permits people or materials to pass in or out of a structure called a pneumatic caisson. The caisson contains compressed air. Air locks are used in digging underwater tunnels or other projects that require caissons. Air pressure in the caisson balances outside water pressure, keeping water out of the working area. Caissons cannot be opened directly to the outer air because the air pressure in the caisson is much greater than the pressure of the outer air. All movements to and from the caisson must take place through an air lock. This is a large airtight chamber equipped with valves. Compressed air is forced into the air lock through these valves until the pressure within the air lock equals the pressure in the caisson. The valves are also used to withdraw compressed air from the air lock until its pressure equals the pressure of the normal outside air.