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largely uninhabited arid region covering some 55,000 square miles (143,000 square km) in central Australia. Situated mainly in the southeastern corner of the Northern Territory, it overlaps into Queensland and South Australia and is bounded by the Finke River (west), the MacDonnell Ranges and Plenty River (north), the Mulligan and Diamantina rivers (east), and the large saline Lake Eyre (south). Sand dunes or ridges, 70-120 feet (20-37 m) high and 1,500 feet (450 m) apart, run parallel from northwest to southeast for distances up to 100 miles (160 km); grass of the genus Triodia (Spinifex) grows between the dune crests. Seasonal rivers running into the Simpson include the Todd, Plenty, Hale, and Hay; ephemeral salt lakes are common throughout its southern sector. There are also mulga trees, a type of acacia scrub, and, immediately following a rare shower, an abundance of wildflowers.