Images from the UN's archive from the 1950s reveal the scale of the covert residence, which included a large living room lined with windows, a compact kitchen, a single bedroom, a tiled bathroom fitted with a bathtub and a private conference room.

Throughout the decades, the apartment was reportedly the backdrop of numerous secret meetings between world leaders and delegates and each Secretary-General is said to have made the space their own. Most notably, Dag Hammarskjöld, who held the position from 1953 to 1961, is said to have adorned the home with borrowed masterpieces from the likes of Picasso and Matisse.

The apartment was eventually converted into offices as part of the Capital Master Plan, a £1.6 million ($2.1bn) renovation of the headquarters, which began in the mid-2000s.

Secret homes, apartments and rooms hiding in unexpected places