A gilgai is a small, ephemeral lake formed from a surface depression in expanding clay soils. Gilgai is also used to refer to the overall micro-relief in such areas, consisting of mounds and depressions. The name comes from an Australian Aboriginal word meaning small water hole.[1] These pools are commonly a few metres across and less than 30 cm (12 in) deep, however in some instances they may reach several metres deep and up to 100 m (330 ft) across. Gilgais are found worldwide wherever cracking clay soils and pronounced wet and dry seasons are present.[1] Gilgais are also called "melonholes, crabholes, hogwallows or puff and shelf formations".[2]