Ancient China was fortunate in having an extensive system of major rivers, especially the Chang Jiang and the Huang He. Although used as waterways, both of these rivers run from west to east, and are largely unpassable from north to south. This was changed with the Grand Canal, which ran from north to south between these two rivers. Not only did this canal make trade and travel easier, but also allowed irrigation in areas far from the river valleys. Begun in the 6th century B.C.E., it was rebuilt 1200 years later, and new canals were added. During the Yuan dynasty (13th century), the system of canals was extended north to the Mongol capital of Beijing, for a total length of 1600 km. Still in use as of the 21st century, the Grand Canal looks to be the longest-lived of the ancient wonders, except for the Egyptian Sphinx.