This important museum is housed in a striking Khmer-style building designed by French art historian George Groslier, originally constructed between 1918 and 1920 as an annex to the École des beaux arts. It was opened by King Sisowath on 13 April 1920 under the name Musée Albert Sarraut. The National Museum houses one of the finest collections of Khmer artefacts in the world, comprising an estimated 10,000 objects, although most are not on display. The majority of artefacts are sandstone from the Angkorian era, but there is also a range of artefacts which date back to the 4th century. During the Pol Pot era the museum fell into disrepair and bats began to roost in the roof space. By the 1990s this colony exceeded 2 million in number, making it the highest known concentration of mammals anywhere in the world. In 1995 the roof was repaired with assistance from the Australian Government through AIDAB and the Australian National Gallery, and a custom-built ‘second ceiling’ installed to protect both artefacts and visitors from falling waste and fleas. However this has failed to live up to expectations and there is currently much debate focusing on the damaging effects of bat guano on the artefacts, as opposed to the income generated by the sale of that substance and the ecological significance of bats as consumers of insects that destroy rice crops.