The town hall with the Mangia tower is seen in Siena, central Italy, January 29, 2016. (Photo by Max Rossi/Reuters)

Two Bulgarian tourists pose in front of the Eiffel Tower in Paris on their last day in the capital on March 24, 2014. (Photo by Franck Fife/AFP Photo)

A general view of Tokyo Tower and the surrounding area in high resolution (~5600x3500 px) on February 10, 2012 in Tokyo, Japan. (Photo by Adam Pretty/Getty Images)

Heather Larsen from Golden, Colorado, USA performs a split while walking the slackfline or highline across two towers in the Tower of David citadel complex overlooking the Old City of Jerusalem 02 May 2016. Larsen is one of the world's top female highliners, or slack liners and walked the lines as part of a Kickstarter campaign for an outdoor product. The tower at left was built by King King Herod some 2000 years ago. (Photo by Jim Hollander/EPA)

Base jumpers leap from the 300-metre high skydeck of Malaysia's landmark Kuala Lumpur Tower against the backdrop of the city's skyline on October 3, 2016 during the annual International KL Tower Base-Jump event. (Photo by Manan Vatsyayana/AFP Photo)

Aviation enthusiast Patrick Wilson 8, from Wetherby looks at an Avro 504 k at “The Shuttlesworth Collection” at Old Warden on July 21, 2014 in Biggleswade, England. Of the 55,000 planes that were manufactured by the Royal Army Corps (RAC) during WWI, only around 20 remain in airworthy condition. Six of these belong to The Shuttleworth Collection at Old Warden, Bedfordshire, making it the most complete collection of original airworthy WWI aircraft in the world. Amongst the collection is the SE5a. The SE5a is a single seater fighter aircraft. It is an original biplane designed by the Royal Aircraft Factory, with its engine built by Wolseley Motors Ltd, and it was issued to 84 Squadron in November 1918. The National Archive in Kew has recently verified that the plane saw action in France with 84 Squadron the day before Armistice, November 10, 1918. (Photo by Dan Kitwood/Getty Images)

A concept illustration of the world's first forest in the sky, the Bosco Verticale green twin towers currently under construction in Milan, Italy. Towering over the city skyline the world's first forest in the sky will be a sight to behold. With tree equal to one hectare of forest spanning 27 floors these 365 and 260 foot emerald twin towers will be home to an astonishing 730 trees, 5,000 shrubs and 11,000 ground cover plants. (Photo by Boeri Studio)

A member of the “Castellers Minyons de Terrassa” adjusts her belt before forming a human tower or ÒcastellÓ during the festival of the patron saint of Barcelona “The Virgin of Mercy” at Sant Jaume square in Barcelona, Spain, September 20, 2015. The formation of human towers is a tradition in Catalonia dating back to the 18th century. (Photo by Susana Vera/Reuters)