The find, which was made by staff from the Mamerki museum in northeast Poland and a group of volunteer historical searchers, is described as the biggest discovery ever made at the 200-hectare forest headquarters.
Bristling with security cameras and surrounded by a solid steel and marble fence with barbed wire, Sobieskiego 100 is just one of several buildings that is occupied by Russia following agreements signed between the People's Republic of Poland and the Soviet Union in the 1970s.
Construction started on Friday of a EUR-199 million tunnel linking two of the islands on which the north-western city of Świnoujście lies.
The surprising find was made after expert cavers began exploring a network of passages that had been constructed during WWII.
From an affably eccentric farmhouse in the village of Poźrzadło to the spectacular greenery and glittering lakes of Łagów ten minutes away, life here is something to be savoured and remembered.
The whereabouts of the complex remains a mystery, but the hunt is on to find it.
The new underground passage will link the Warsaw HUB to a metro station.
Once completed the tunnel near the Czech border will be the longest in Poland.
The stunning discovery was made in the residential neighbourhood of Żoliborz in northern Warsaw where the 300-year-old church was built as a summer residence for Queen Marie Casimire.