F-4 Object / Rákosi Bunker, Hungary

F-4 Object (Hungarian: F-4 Objektum or Rákosi Bunker) is a secretive Cold War atomic shelter hidden beneath the streets of Budapest, Hungary. Built in the early 1950s/1960s under absolute secrecy, this vast subterranean shelter was constructed for Hungary’s top leadership. Its purpose, to shield the top-ranking communist party officials from nuclear attack. Its strategic position and scale make it one of the largest and (once) most secret fallout shelters in the country. The bunker is a relic, a reminder of paranoia and preparations during the Cold War. Built for nuclear strikes that thankfully never came, it remains one of Budapest’s most fascinating and hidden relics.

I visited this unique location during my 2024 road trip around Hungary. After checking for access the night before while passing through the area in search of food and a rock bar, I was thrilled to discover that my research had paid off. I had found an entry portal! Even better, it was open. I returned the following night, armed with camera gear and multiple backup torches, ready for an underground adventure. Slipping underground and descending the stairs, I could hardly believe I was actually here. I had dreamed of exploring this mega bunker for years.

As it turns out, I was just in time. Only a few months later, the entry portal was sealed with tonnes of rubble. With an unknown number of entry points, I am sure it is only a matter of time before the bunker is cracked once again!

History of the Rákosi Bunker (F-4 Objektum)

Construction began in the early 1950s under the rule of Mátyás Rákosi, Hungary’s Stalinist leader. The bunker was completed in 1962 or 1963. At this time the Cold War was in full swing. The Soviet-aligned government in Hungary began to prepare their country for the worst: direct atomic strikes on Budapest and surrounding areas. Object F-4 was designed to serve as a nuclear-proof shelter, protecting top government members from the Central Committee of the Hungarian Working People’s Party during a potential conflict.

Its existence was a closely guarded secret. Workers were reportedly blindfolded during parts of the construction, and many believed they were simply expanding sections of the Budapest Metro. The project was directed by Hungary’s Ministry of Interior, and much of the planning and execution was based on Soviet bunker designs.

In the end, the bunker was never activated for its intended purpose. It remained on dutiful standby throughout the Cold War, maintained and ready for nuclear war to break out. After the Soviet Union fell and the threat of nuclear war diminished, the existence of this once top-secret bunker gradually became public knowledge.

Today, the Rákosi Bunker remains out-of-sight-out-of-mind for the thousands of inhabitants in Budapest who live above the labyrinth of tunnels.

Structure and Layout of the Mysterious F-4 Object

The Rákosi Bunker lies beneath central Budapest, situated around 45 metres underground. Its structural form is reinforced concrete, with narrow access tunnels leading into a more expansive central complex. Underground passageways connect the bunker to several key buildings above ground, including the former Hungarian Working People’s Party HQ. Undoubtedly the layout would enable the secret escape of the officials to the safety of the bunker, without public visibility.

• The construction would cost an estimated HUF 6–8 billion (EUR 150–200 million) at today’s exchange rate
• Area of 3,500 – 3,800 square metres 45-50 metres below the surface
• Several kilometres long, an approximate “H” shape in plan view
• Heavy blast doors, marking critical transitions between sections
• Power rooms and diesel generators, 30 kW production potential
• Systems for air filtration and ventilation are in place. Capacity of 4,000 cubic metres, capable of scrubbing the air of radioactive dust
• 150 cubic metre capacity water reservoir
• Living quarters with bunk beds and locker areas
• Communication rooms intended for secure transmissions
• Storage areas

Inside, the bunker comprises multiple chambers. These include:

One of the most significant features was its link to the Budapest Metro, specifically Metro Line M2. However, these links were secret and were not intended for civilian use; this was strictly a communist party members-only bunker. Initially the subterranean connection allowed discreet movement of personnel and materials during bunker construction. It also provides an additional underground escape route should the surface portals be compromised. There are rumours that government officials could potentially use the metro tunnels to escape the city, rendezvousing with extraction teams at an unknown extraction point.

The mega atomic bunker today

Presently, access to the Rákosi Bunker is highly restricted. The site is not officially open to the public, and formal tours do not operate. Nowadays it is owned by BKV (Budapest Transport Company), who make regular checks of the generators and ventilation system. The lights are kept on in these areas, burning deep below the streets in this unseen Cold War relic.