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)

A girl attends a performance hosted by the Ministry of the Interior that includes police, border guards, and rescuers celebrating International Children's Day in Kyiv, Ukraine on June 1, 2021. (Photo by Gleb Garanich/Reuters)

Border Security Force (BSF) soldiers ride their camels as they take part in a rehearsal for a road show ahead of the visit of U.S. President Donald Trump, in Ahmedabad, India, February 21, 2020. (Photo by Amit Dave/Reuters)

People clash with Venezuelan National Guards as they try to cross the border to Colombia over the Francisco de Paula Santander international bridge in Urena, Venezuela December 18, 2016. (Photo by Carlos Eduardo Ramirez/Reuters)

A policeman chases away a farmer during a protest against the newly passed farm bills at Singhu border near Delhi, India, November 27, 2020. (Photo by Danish Siddiqui/Reuters)

An Israeli army officer gives explanations to journalists during an army organised tour in a tunnel said to be used by Palestinian militants for cross-border attacks, July 25, 2014. (Photo by Jack Guez/Reuters)

Autumn colors border a road leading through a small valley near Blankenburg am Harz, Germany, Wednesday, November 1, 2023. (Photo by Matthias Schrader/AP Photo)

A woman is supported by two men while crossing a river, north of Idomeni, Greece, as migrants attempt to reach Macedonia on a route that would bypass the border fence, Monday, March 14, 2016. Hundreds of migrants and refugees walked out of an overcrowded camp on the Greek-Macedonian border Monday, determined to use a dangerous crossing to head north. (Photo by Vadim Ghirda/AP Photo)