artesian depression in Western Australia and South Australia, Australia. Sloping southward to the Great Australian Bight and underlying the enormous limestone waste of the Nullarbor Plain, its area is about 69,500 sq mi (180,000 sq km). Composed of two main aquifers, the upper layer of the basin is a sequence of limestones of Tertiary age and the deeper layer is composed of sandstone of Cretaceous age. Surface water percolating down through the limestone of the basin has created subterranean caves and tunnels. Small amounts of groundwater can be tapped through bores. The name Eucla is derived from the Aboriginal words yer, meaning “bright”; and coloya, meaning “fire.”