The Albuquerque Basin is one of the largest (8,000 km2, 3,060 mi2) and deepest basins (4,407-6,592 m, 14,500-21,600 ft) of the Rio Grande rift. This basin contains the largest metropolitan area in New Mexico. Until 2008, this region relied entirely on groundwater for its water supply. This sole reliance on groundwater resulted from an earlier view that Albuquerque lay on top of the subterranean equivalent of a vast underground lake that would take centuries to exploit. Since the 1960s, the City of Albuquerque had little reason to be concerned about its water supply because wells drilled in the northeast and southeast heights yielded large quantities of potable groundwater. The view of plentiful groundwater was essentially unchallenged until the late 1980s, when water level declines near Coronado Center provoked exploration of the deeper aquifer. Results of the deep aquifer test wells led to reassessment of the regional aquifer and the Middle Rio Grande Basin Project of the late 1990s.