lake in central Mexico. Originally one of the five lakes contained in Anáhuac, or the Valley of Mexico, Texcoco has been drained via channels and a tunnel to the Pánuco River since the early 17th century, until it now occupies only a small area surrounded by salt marshes 2 ¹/₂ mi (4 km) east of Mexico City. Tenochtitlán, the Aztec capital, captured in 1521 by the Spanish conquistador Hernán Cortés, stood on islands in old Texcoco, connected to the mainland by causeways. It was hoped that vast areas of rich farmlands would be made available by draining the lake, but the soils proved too saline for cultivation. In addition, the surface of the lake bed has dropped and the buildings on it have settled, and clouds of dust are occasionally blown over the urban area, contributing to the extreme air pollution of Mexico City.