historical and cultural region of Mexico. The heartland of Aztec Mexico, Anáhuac (Nahuatl: “Land on the Edge of the Water”) designated that part of New Spain that became independent Mexico in 1821. The original Anáhuac of the Aztecs was the part of the Mesa Central of Mexico, an area about 50 miles (80 km) long by 30 miles (50 km) wide, with an average elevation of 7,500 feet (2,300 metres). When the Spaniards arrived in 1519, the valley contained five interlocking lakes (Zumpango, Xaltocan, Texcoco, Xochimilco, and Chalco), in the midst of which stood Tenochtitlán (now Mexico City), the Aztec capital, linked to the mainland by three long causeways. In 1607–08 much of the water was drained off to the Pánuco River system by a ditch and tunnel, and in the 20th century Lake Texcoco (Texcoco, Lake) was further emptied. Its salt marshes remain, as do Xochimilco's floating gardens, but the vast water area to the east of downtown Mexico City is gone. Saline deposits make the reclaimed land unfit for agriculture, and the dry lake beds contribute to the air pollution of Mexico City. The lake beds are unstable, and parts of the city have been sinking.