Built between 1953 and 1979 inside a Zlatar hill at the southern foothill of the Bjelašnica Mountain, the ARK is the largest nuclear shelter, and one of the largest underground facilities, ever built in the former Yugoslavia. For over a decade after its completion, it was the most secret military installation in the country. All construction workers were carefully selected and vetted, signing a contract of silence, and all staff members held the highest security clearance.