Running roughly parallel with the Danube on the city’s Pest side, Metro line 3 spans 17 kilometres between north and south Budapest, serving a total of 20 stations and transports nearly 500,000 passengers every day, making it the longest and busiest line of the city’s subway system. Sitting in a metro carriage, you are only touching the tip of the iceberg as more much is lurking here than you would imagine. What is deep, deep down below Pest's pavements, what is behind tempting ‘no entry’ signs and could the underground even save lives? We Love Budapest took a four-hour night tour around the tube system to find out the answers to these questions. See our pictures below.

There is something intriguing about the Budapest underground; once you leave behind the beautiful architecture above ground, you arrive at the dimly lit cosmos of Communist craft and decaying concrete. It is a surreally stunning part of town that captures the imagination. If you happen to have seen all-time favourite Hungarian film, Kontroll, you have some idea. This year, Budapest’s public Transit Company, BKK, turned 50, and organised a special exclusive tour for the press, presenting the underground system as never before.

The tour started at 10pm from busy Deák tér metro station. Our guide behind the ‘no entry’ and ‘danger’ signs was Zoltán Vajda, the head of infrastructural engineering at BKK. The first stop was at the main ventilator, which lies 30 metres roughly below Hotel Kempinski, from where a massive vent and a steel pipe lead to the surface. This ventilator provides stations with fresh air.

When the metro system was built during the Cold War, the country was still dreading the prospect of atomic disaster. Therefore, most sections of the metro system were designed to also fulfil the function of a nuclear-proof hide-out in case of emergency.

The atmosphere down here is, indeed, post-apocalyptic, where poison gas filters are marked with Cyrillic lettering and heavy iron locks can seal every exit leading to the surface. For safety reasons, the metro system can also be divided into sections; metro line 3 has nine, while metro 2 has three individual segments, all with their own electricity, water and air supply. In case of emergency, one sector can cover for its two neighbours, while in case of an air raid, they can immediately be separated from each other. This means that people down here would not suffocate even if the whole Hotel Kempinski collapsed, as the ventilators would reach above the level of the ruins and the ten-centimetre-wide steel pipe can hold the weight of the whole building. In case of danger, the tunnel system can accommodate 220,000 people.

After midnight when the last metro leaves, electricity in the tracks is turned off and instead of the usual carriages, small service trains circulate, maintaining the tracks.

In the tunnel, a memorial plaque marks the water level of the 1991 flood that broke through into the subway system at Astoria station on metro 2. Many staff members had to be rescued – some refrain from using the underground system even today – and due to significant damage, water had to be drained manually. Marble features of Astoria station were destroyed and restoration of the escalators took several months.

At 16 floors below the surface, there is an immense nuclear-proof bunker called the Rákósi bunker, named after Hungarian Communist leader Mátyás Rákosi and built exclusively for Party members under top-secret conditions in 1951. Workers from the provinces were employed and, as it is easy to become disoriented underground anyway, they had no idea which part of town they were under. They believed that they were working on the metro. The bunker was never really finished and the fate of this 350,000-square-metre, two-storey space is uncertain to this day. Plans for its use in the past included everything from a mushroom farm to a disco and a super-secret computer-server room. As of now though, it lies empty.