David Rawcliffe, house and monument steward at the National Trust's Chedworth Roman Villa cleans a Roman mosiac in the new environmentally-controlled conservation shelter on March 19, 2012 near Cirencester, England. Opened this month, the multi-million pound building allows visitors to walk on suspended walkways just above the 1600-year-old Roman floors and to see recently excavated mosaics not seen for 150 years, with more to be uncovered over the coming year. (Photo by Matt Cardy/Getty Images)

A lithographic sketch, purportedly by Beatles star John Lennon, shows his wife Yoko Ono in a sexual context within a document released by The National Archives March 23, 2004 in London. The document, which has been classified since 1970, contains sketches and letters which were seized by police to form part of the evidence for the indecency trial against the London Art Gallery which had been displaying the material. (Photo by Ian Waldie/Getty Images)

People perform during celebrations to commemorate the 70th anniversary of the establishment of the Vietnam Public Security police force at the National Convention Center in Hanoi August 18, 2015. The Vietnam Police force was founded on August 19, 1945, the day of the anti-colonial people's uprising known as the August Revolution. (Photo by Reuters/Kham)

Emma Coburn (L) and Colleen Quigley of the U.S. look at the scoreboard after competing in the women's 3,000 metres steeplechase final during the 15th IAAF World Championships at the National Stadium in Beijing, China August 26, 2015. (Photo by David Gray/Reuters)

A Nepalese boy gets ready to perform a stick dance during a rally held to pay tribute to the victims of an earthquake, in Bhaktapur, Nepal, Wednesday, September 2, 2015. Two powerful earthquakes in April and May devastated the Himalayan nation killing more than 8,800 people. (Photo by Niranjan Shrestha/AP Photo)

Doris Dean of Chicago, Ill., sits with her gun shortly after she won a 50 meter, 40-shot event for women with a perfect 400 score at the National Rifle Association (NRA) matches at Camp Perry, near Toledo, Ohio, September 7, 1941. (Photo by AP Photo/DW)

A tourist stands at an edge of the singing sand, the 150-metre-high by three-kilometre-long dune that generates a low-pitched, organ-like rumble in dry weather, in Altyn-Emel national park in Almaty region, Kazakhstan, May 12, 2016. (Photo by Shamil Zhumatov/Reuters)

A paper mache pig meant to mock Donald Trump sits on display at a protest against the Republican presidential candidate before the start of the Republican National Convention in Cleveland, Ohio, USA, 17 July 2016. Numerous protest groups are expected in Cleveland throughout the four-day-long convention. (Photo by Jim Lo Scalzo/EPA)