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] The pools are commonly a few metres across and less than 30 cm (12 in) deep but, in some instances, they may be several metres deep and up to 100 m (330 ft) across. Gilgais are found worldwide wherever there are cracking clay soils and marked wet and dry seasons.^[1] Gilgais are also called melonholes, crabholes, hogwallows, or puff and shelf formations.^[2]

A gilgai landscape, Queensland

Gilgais are thought^[1] to form in vertisols through repeated cycles of swelling when wet and subsequent shrinkage upon drying. That action, known as argillipedoturbation, causes the soil to crack when dry, allowing loose soil material to fill the cracks. When the soil swells upon subsequent re-wetting, the soil pressure cannot be dispersed into the now-full cracks and the soil is forced sideways causing a mound to form between cracks and a depression to form at the location of the crack. That process is then exaggerated as the depressions hold water and thus become wetter and swell more than the mounds, causing even greater shrinkage and cracking. In addition, the cracks channel water deeply into the soil, causing even greater swelling and subsequent cracking of the depression areas. Each cycle of swelling, shrinkage and cracking becomes more exaggerated and the landscape eventually becomes covered by a repeated pattern of mounds and depressions. The depressions hold surface water during wet seasons.

Australia has an abundance of cracking clay soils and large areas dominated by pronounced wet and dry seasons, providing ideal circumstances for gilgais to form. Central Russia and several parts of the United States, including South Dakota, Mississippi, and Texas^[3] also host the formations.^[1] Gilgais are structurally similar to the patterned ground of frigid regions, but periglacial soil polygons are formed instead, by repeated freeze-thaw cycles.

Gilgais were an important water source for Indigenous Australians, enabling them to forage over areas that lacked permanent water. Similarly, they allowed the stock of early Australian pastoralists to graze those areas. The introduction of wells and pumps reduced the value of gilgais to humans as a source of water. Many farmers now generally regard them as a nuisance. The movement of soil associated with gilgai formation damages infrastructure, including building foundations, roads and railway lines, and the undulations interfere with crop harvesting. The presence of seasonal water in grazing land makes it more difficult to control stock and provides a water supply for feral animals and kangaroos.

Gilgais remain of great ecological significance as a source of water for animal and plant life. Crayfish burrow in the wet basins and ants build up the mounds, magnifying the formations through bioturbation.^[4]

• Mima mound
• Bay of Biscay soil

• Alekseeva, T.V. & Alekseev, A.O. 1997 "Clay mineralogy and organization of finely dispersed material of gilgai soils (Stavropol Krai)" Eurasian soil science 30:8 867-876
• Anthony J. Parsons, A. D. Abrahams, ed. (2009). Geomorphology of Desert Environments. Springer. pp. 109–112. ISBN 978-1-4020-5718-2.
• Andrew S. Goudie (2013). Arid and Semi-Arid Geomorphology. Cambridge University Press. p. 105. ISBN 978-1-107-00554-9.
• Beckmann, G. G., Thompson, C. H., and B. R. Richards. 1984. Relationships of soil layers below gilgai in black earths. In J. W. McGarity, E. H. Hoult and H. B. So (eds.) The Properties and Utilization of Cracking Clay Soils. Reviews in Rural Science no. 5. Armidale, NSW, University of New England, pp. 64–72.
• Costin, A. B. 1955a. A note on gilgaies and frost soils. Journal of Soil Science 6: 32–34.
• Hallsworth, E. G. and G. G. Beckmann. 1969. Gilgai in the Quaternary. Soil Science 107: 409–420.
• Hallsworth, E. G., Robertson, G. K., and F. R. Gibbons. 1955. Studies in pedogenesis in New South Wales. VII. The ‘‘gilgai” soils. Journal of Soil Science 6: 1–31.
• Jensen, H. I. 1911. The nature and origin of gilgai country. Proceedings of the Royal Society. NSW 45: 337–358
• Knight, M. J. 1980. Structural analysis and mechanical origins of gilgai at Boorook, Victoria, Australia. Geoderma 23: 245–283 .
• McManus, K. 1999 "Mound Theory, Gilgai and PSD Analysis" Proceeding, 8th Annual Australia and New Zealand Conference on Geomechanics, Hobart
• Ollier, C. D. 1966. Desert gilgai. Nature 212: 581–583.
• Paton, T. R. 1974. Origin and terminology for gilgai in Australia. Geoderma 11: 221–242.
• Stephen, I., Bellis, E., and A. Muir. 1956. Gilgai phenomena in tropical black clays of Kenya. Journal of Soil Science 1–9.
• White, E. M. and R. G. Bonestell. 1960. Some gilgaied soils in South Dakota. Soil Science Society of America Proceedings 24:305–309.
• Wilson, J.W. 1964 "Vegetation patterns in an unusual gilgai soil in New South Wales," The Journal of Ecology, 52:2 379-389