Built between 1953 and 1979 inside the 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.
Visible from the outside of the complex are three nondescript houses, not marked on any map, surrounded by natural settings and located along the Neretva River. For nearly 50 years, the houses concealed entrances to the third largest military facility in Yugoslavia (after the underground Željava Air Base near Bihać and the Lora naval facility in Split, Croatia). The houses consist of a control residence, a building containing secure communication links, and the facility where technical support personnel would be located. Along a corridor located in the first house, three 1.20 meter-thick metal doors protect a tunnel leading to the heart of the shelter, which is 280 meters deep and 202 meters long.
The underground horseshoe-shaped structure, built to withstand a 20-kiloton nuclear blast, is divided into interconnected 12 blocks, the most important being Blocks 6 (communication), 8 (Tito’s block), 9 (air filtration), 10 (fuel storage), and 11 (water storage). Designed to protect and accommodate up to 350 people for up to six months, the bunker covers an area of 6,854 m² and contains over 100 rooms, including dormitories, two large conference rooms, five operation centers with direct phone links to the Presidency, two kitchens, five large toilets, a cryptography center, cable television access, and a fully equipped hospital operating room. President Tito’s residence consists of five rooms: one for his secretary and party leaders, Tito’s office, his bedroom, from which one could enter the room of Yugoslav First Lady Jovanka Broz, and a relaxation room.
Within the structure are 21 large systems and maintenance rooms with containers capable of holding 50 tons of oil, climate control systems, 170 cubic-meter water tanks, and running water drawn from natural wells located within the mountain. Each system was equipped with a back-up in case the primary failed. The temperature in the bunker is between 21 and 23 degrees Celsius, with humidity between 60% and 70%, representing near-optimal subterranean living conditions. The complex is lit by approximately 6,000 neon bulbs.