Transporting large quantities of water by tanker has become more common in recent years, as the unit cost of such transport has declined. Moreover, the current glut in the oil market has put a good proportion of the tanker fleet out of operation. These developments have provided an incentive for oil shippers to transport water to arid areas on their return voyages in tankers especially equipped with ballast tanks, but this has still not been done on a long-term or large scale basis.