The largest-ever preservation project at Dunhuang's Mogao Grottoes, a collection of ancient Buddhist cave artworks, has been completed with the official opening of an affiliated visitor center on Wednesday. Occupying the caves on a 1,700-meter-long cliff, the Mogao Grottoes has some of the world's finest Buddhist frescos and sculptures, which were made as early as 1,600 years ago. It was listed as a UNESCO World Heritage Site in 1987. Since late 2008, a series of preservation efforts have taken place at the Mogao Grottoes, including cave consolidations, redesigned tourist footways, security and sandstorm-protective facilities, as well as the newly-opened visitor center, which shows digital images of the caves. Starting from Sept 11, visitors are required to undergo an online reservation process in order to visit the Mogao Grottoes