Built in 1AD, this 2000 year old Roman aquaduct is 2,500 feet long, 100 feet high, has 118 arches-- and was made without any mortar. It still works and is considered the emergency water supply for the city (it was used as the primary water supply until the late 19th century). The acqueduct channeled the Río Frío through a series of tanks to the ancient city of Segobriga, an important Roman military base.