Timbuktu: A UN peacekeeper from Burkina Faso stands guard at the Djinguereber mosque, built in the 14th century, during a visit by a UN delegation on election day in Timbuktu, Mali, July 28, 2013. (Photo by Joe Penney/Reuters)

Wieliczka Salt Mine: Part of the salt sculpture The Last Supper is pictured at The Saint Kinga's Chapel in the Wieliczka Salt Mine near Krakow, southern Poland, December 15, 2011. According to the Supervisory Board of the Wieliczka Salt Mine, the historic mine extends for a total of about 300 km (186 miles) and functioned continuously since the Middle Ages until 1996 when the salt bed ceased to be exploited completely. (Photo by Kacper Pempel/Reuters)

Sydney Opera House: The building is reflected in a harbourside hotel window in The Rocks district of Sydney, May 7, 2014. Since it's completion in 1973, the iconic Australian landmark has attracted millions of visitors and in 2007 became a UNESCO World Heritage site. (Photo by Jason Reed/Reuters)

Goree Island: A man is silhouetted in the “Door of No Return” at the House of Slaves museum on Goree Island near Senegal's capital Dakar, March 16, 2007. Millions of Africans were shipped from places like this and from the whitewashed fort in Elmina, Ghana, to a life of slavery in Brazil, the Caribbean and America. (Photo by Finbarr O'Reilly/Reuters)

Rila Monastery: A general view of the main church at the Rila Monastery, southwest from Sofia on June 18, 2003. Rila Monastery, the biggest Bulgarian monastery was founded in the l0th century by the Bulgarian monk St. John of Rila and was rebuilt in the l3th -l4th century. In the l5th century, when Bulgaria fell to the Ottoman Turks, the monastery was abandoned for a short time but in the second half of the same century it was restored to a new life. (Photo by Oleg Popov/Reuters)

Red Square and the Kremlin: Russian fighter jets fly in formation over Red Square and the Kremlin during a military parade dress rehearsal in Moscow May 6, 2010. (Photo by Denis Sinyakov/Reuters)

Stonehenge: A meteor streaks past stars in the night sky over Stonehenge in Salisbury Plain, southern England August 12, 2010. (Photo by Kieran Doherty/Reuters)

Rio de Janeiro: An aerial view of the famous Christ the Redeemer atop of Corcovado mountain in Rio de Janeiro, January 12, 2011. (Photo by Bruno Domingos/Reuters)

Potala Palace: A Tibetan pilgrim prays in front of the Potala Palace in Lhasa in Tibet Autonomous Region July 5, 2006. (Photo by Claro Cortes IV/Reuters)

Acropolis: A tourist stands in front of the Parthenon Temple at the archaeological site of the Acropolis Hill in Athens November 3, 2011. (Photo by John Kolesidis/Reuters)

Sukhothai: A general view of a Buddha statue is seen at the Sukhothai historical park in Sukhothai September 17, 2009. (Photo by Ajay Kamalakaran/Reuters)

Sigiriya: Women leave Sigiriya, an ancient rock fortress and palace ruin, in Matale District, east of Colombo, June 20, 2010. (Photo by Andrew Caballero-Reynolds/Reuters)

Petra: A child walks in the amphitheatre which was first constructed by the Nabataeans in the early 1st century AD and refurbished by the Romans soon after their conquest in 106 AD, in the ancient city of Petra April 7, 2008. (Photo by Erik de Castro/Reuters)

Luang Prabang: Tourists disembark from a slow boat to Luang Prabang as it stops at Pak Beng, Laos October 25, 2007. (Photo by Tim Chong/Reuters)

Jantar Mantar: Tourists visit Jantar Mantar, a collection of architectural astronomical instruments, in Jaipur, capital of India's desert state of Rajasthan January 12, 2009. The name Jantar Mantar is derived from instrument for formula on context calculation as this observatory consists of fourteen major geometric devices for measuring time, predicting eclipses, tracking stars in their orbits, ascertaining the declinations of planets, and determining the celestial altitudes and related ephemeredes. (Photo by Vijay Mathur/Reuters)

Pyu ancient cities: A couple passes time at an ancient pagoda at the ancient city of Sri Ksetra, outside Pyay, Bago division, August 24, 2014. Sri Ksetra, an ancient Pyu city built over 2,000 years ago, has been included in the UNESCO world heritage list. (Photo by Soe Zeya Tun/Reuters)

Ephesus: A woman walks in the ancient Ephesus ruins near Turkey's western coastal city of Izmir September 11, 2009. The ruins of Ephesus, one of the country's important archaeological sites of succession of great ancient civilizations, is a popular tourist destination. (Photo by Osman Orsal/Reuters)

The Alamo: Visitors walk at the entrance to the Alamo, the most-visited tourist site in the state, in San Antonio, Texas March 2, 2015. (Photo by Lisa Maria Garza/Reuters)

Versailles Palace: The Chateau de Versailles is seen in an aerial view outside Paris July 14, 2011. (Photo by Charles Platiau/Reuters)

Imperial Palaces: Tourists inside the Forbidden City in Beijing August 23, 2008. (Photo by Kevin Coombs/Reuters)