For 2,000 years, Aqueducto de Segovia has been conveying drinking water from the Frio River to Segovia, approximately 18 kilometers away. Built under the reign of Roman emperor Trajan, the aqueduct is one of the most intact and best-preserved Roman engineering masterpieces. Roman engineers built the channel of the aqueduct with an average one percent gradient over its whole length. To span the River Clamores outside Segovia and reach the city, they constructed the massive 270-meter-long aqueduct using over 24,000 Guadarrama granite blocks carefully fitted together to support themselves without the use of mortar. The aqueduct in the center of Segovia consists of a double-tiered section that rises over 28 meters above the Plaza Azoguejo. The doubled tiers were necessary to maintain an even water flow gradient, even though the ground below slopes away.