‘Extreme lakes’ describes a range of lakes that are of great scientific interest because of their unusual physical, chemical, and biological features. Salt water lakes occur in many parts of the world and are often highly productive, with a simplified food chain supporting large flocks of birds. Polar and alpine lakes are strongly affected by snow and ice, and are therefore sensitive to small changes in temperature across the freezing–melting threshold of water. Other extreme lakes include those that have acid and alkaline waters; geothermal hot-water lakes, and water bodies that periodically erupt, disgorging liquid and gas. In some, only the hardiest of ‘extreme-loving’ microbes can survive and grow.