Air around the lung
There is normally no air in the thin space between the rib cage and the lungs.
Normally the negative pressure inside the ribcage keeps the lung inflated with the lung gliding against the cage. The lungs expand as the rib cage moves. If there is a hole in the lung or if there is a stab from the outside to inside of the rib cage, the pressure rises and the lung collapses - a pneumothorax.