It has been known for over 300 years that the anatomy of the elephant lung is unique among mammals in that the pleural cavity is obliterated by connective tissue. However no satisfactory explanation has been advanced. Recent studies suggest that the elephant has an aquatic ancestry and the trunk may have developed for snorkeling. In addition, the modern day elephant is the only mammal that can remain submerged far below the surface of the water while snorkeling. The resulting differences of pressures within the thorax mean that the small blood vessels of the pleura are in great danger of rupturing or causing severe edema. The same distribution of pressures occurs when the animal raises water inside its trunk prior to drinking although in this case the pressure differences are relatively short-lived. Evolution has provided a remarkable solution to this problem by replacing the normally delicate parietal and visceral pleurae by dense connective tissue, and separating the two pleurae by loose connective tissue to allow some sliding movement.