Free air beneath the diaphragm can be an iatrogenic phenomenon as following laproscopy. Perforated ulcers or diverticula can lead to this condition as can puncture injuries to the abdomen or bowel.
(Place mouse over image for labels)
The white arrows delineate the edge of the lung, the visceral pleura. White arrowheads designate the superior margin of the diaphragm, parietal pleura.