The first stage of the renovation of an ancient subterranean irrigation system in northwest China's Xinjiang Uygur Autonomous Region has been completed, local authorities said Saturday.

The karez irrigation system was built 2,000 years ago. Conveying water from deep underground to the surface, the karez wells are considered - along with the Great Wall and the Grand Canal - one of the China's greatest surviving antique man-made structures.

The renovation of 31 karez wells was completed in mid October and a second round of work will start on 52 wells in November, said Zhao Qiang, an official with the Turpan Prefecture Cultural Relics Bureau.

The renovations started in December 2009 after the State Administration of Cultural Heritage earmarked 16 million yuan (2.4 million U.S. dollars) for the still-in-use wells in arid Xinjiang.

"We first clean up the sludge, stones and bricks in wells where cave-ins have occurred. Then we repair and strengthen the damaged sections," said Nurjohn Kader, a worker on the renovation project.

The karez wells are vertical shafts and subterranean canals that surface in the form of ditches and small ponds. The canals are supplied with water from the melted ice and snow in Xinjiang's Tianshan Mountains.

Karez wells are also found in nearby countries - Afghanistan, Iran and Pakistan. In China, they can also be found in the provinces of Gansu, Shaanxi and Shanxi.

Traversing over 5,000 kilometers, the karez wells are also known as the "Great Underground Wall."