Željava Air Base, situated on the border between Croatia and Bosnia and Herzegovina under Plješevica Mountain, near the city of Bihać, Bosnia, was the largest underground airport and military air base in the former Yugoslavia, and one of the largest in Europe. (Photo by Thomas Windisch/Exclusivepix Media)

An animatronic polar bear visits the London Underground on January 23, 2015, to mark the launch of Fortitude, Sky Atlantic&Otilde's new drama starring Stanley Tucci, Michael Gambon and Christopher Ecclestone, which premieres on Thursday 29th January at 9pm. A team of 19 Hollywood prop specialists spent six weeks designing and building the realistic replica animal with six to eight people working on the costume at any one time, using over 60 different materials. (Photo by David Parry/PA Wire)

A snowflake rests on black velvet after a snowfall in Knoxville on Tuesday, January 28, 2014. (Photo by Adam Lau/News Sentinel)

Actress Liv Tyler, 40, stripped in shots for the Triumph Essence's Autumn/Winter 2017 campaign in November, 2017. The Essence line is Triumph’s premium offering, with this season taking on the theme of “opulent Art Nouveau”, featuring designs in velvet embellished mesh, Leavers lace and silk satin. (Photo by Rankin/The Mega Agency)

Staff at a Scottish farm say they are “absolutely delighted” to have welcomed a baby alpaca into the world this morning, June 6, 2019. Mum, Nunavut, gave birth to the baby boy huacaya alpaca, which has not yet been named, weighing 9.6kg. Stuart Ramsay, the owner of Velvet Hall Alpacas, in Innerleithen, Scottish Borders said he was surprised when the baby was born an “unusual rose grey colour”. (Photo by South West News Service)

A simulated casualty is evacuated during an emergency services exercise at the disused Aldwych underground station on February 22, 2012 in London, England. Operation Forward Defensive aims to test the response to a terrorist incident on the London Underground network during the 2012 Olympic Games. (Photo by Peter Macdiarmid/Getty Images)

Tube passengers take a selfie next to a London underground roundel designed for the launching of the Night Tubes on display at Oxford Circus underground station in London, Britain, 19 August 2016. The long delayed Night Tubes service launched on 19 August. The 24-hour service will operate on Fridays and Saturdays. (Photo by Hannah Mckay/EPA)

People stand under a giant Czech flag as they arrive to light candles at the base of a statue of St. Wenceslas to commemorate the death of former Czech President Vaclav Havel on December 18, 2011 in Prague, Czech Republic. Havel, who led the Velvet Revolution that forced communist rule in Czechoslovakia to crumble in 1989, died in the early morning today in his sleep at the age of 75. (Photo by Sean Gallup/Getty Images)