This extreme habitat is located in the intermediate zone between the coast and the Andes, between Arica and Tocopilla where the annual precipitation is between 0.5 mm and 16 mm. The reasons for this extreme aridity are complex and the result of several inter-related factors. These include temperature inversion driven by the cool north flowing Humboldt current and the Southeast Pacific Subtropical Anticyclone. The latter is a high pressure centre, located off the coasts of Chile and Peru, which causes a thermal inversion that prevents the clouds that form over the sea from developing in height, thus preventing precipitation. Furthermore, the considerable distance from the Atlantic Ocean and the rain-shadow effect at the western flank of the Andes, results in little moisture arriving from the Atlantic, except in the Altiplano where it rains in summer. The only available water is from aquifers fed from the Andes. Typically, the hyper-arid desert is largely devoid of vegetation except for scattered shrubs, cacti, perennials and annuals, all of which are highly adapted to be either drought-evaders or drought-tolerant. Trees such as Prosopis tamarugo survive by taking advantage of relatively shallow (5-15 m) underwater aquifers.