The basin is still gradually sinking, and still gradually accumulating sediment. For many millions of years, the Lake Eyre Basin was well supplied with water and largely forested. About 20 million years ago, large shallow lakes formed, covering much of the area for about 10 million years. From that time on, as Australia drifted further north and the climate became gradually more arid, the lakes and floodplains started to dry. Only in the last 2.6 million years did the onset of the ice ages bring about the present climatic regime and the consequent fairly rapid desertification of the area. The basin covers just under one-sixth of all Australia and is the largest endorheic basin in Australia and amongst the largest in the world, covering about , including much of inland Queensland, large portions of South Australia and the Northern Territory, and a part of western New South Wales. The basin is also one of the largest, least-developed arid zone basins with a high degree of variability anywhere. It supports about 60,000 people and a large amount of wildlife, and has no major irrigation, diversions or flood-plain developments. Low density grazing is the major land use, occupying 82% of the total land within the basin. Significant minerals deposits such as oil and natural gas, including Australia's most significant onshore petroleum reserves, are found within the basin. The mining and petroleum industries account for the greatest economic activity in the Lake Eyre Basin. Opals, coal, phosphate, gypsum and uranium are also mined from the basin. In 2009, the Queensland Environmental Protection Agency confirmed that heavy metals from mining operations near Mount Isa had entered the upper reaches of the Georgina River. The spill has the potential to contaminate parts of the basin as far south as Lake Eyre. During years of especially high rainfall, all the riverbeds in this vast, mostly flat, arid and semi-arid area lead inland (not towards the sea) towards Lake Eyre in central South Australia. Lake Eyre itself lies approximately below sea level, and usually contains only salt. In flood years, it fills and, for a short time, undergoes a period of rapid growth and fertility: long-dormant marine creatures multiply and large flocks of waterfowl arrive to feed and raise their young before the waters evaporate once more. The annual mean runoff in the Lake Eyre Basin is lowest of any of the world's major drainage basins. None of the creeks and rivers in the Lake Eyre Basin are permanent: they flow only after heavy rain – a rare to very rare event in the arid interior of Australia.