The F4 Object is a Cold War nuclear shelter hidden deep below central Budapest, between Kossuth tér and Szabadság tér. Built about 45 to 50 metres underground, it was designed as a protected command space with links to the metro and nearby government buildings.
Access is specialised rather than casual, but the site is one of Budapest's stranger underground Cold War remnants. It is most interesting if you are already drawn to the city's bunkers, tunnels, and political history.