Located about 27 m (89 ft) below base level,[80] it measures roughly 16 cubits (8.4 m; 27.5 ft) north-south by 27 cubits (14.1 m; 46.4 ft) east-west, with an approximate height of 4 m (13 ft).
The western half of the room, apart from the ceiling, is unfinished, with trenches left behind by the quarry-men running east to west. A niche was cut into the northern half of the west wall. The only access, through the Descending Passage, lies on the eastern end of the north wall.
Although seemingly known in antiquity, according to Herodotus and later authors, its existence had been forgotten in the Middle Ages until rediscovery in 1817, when Giovanni Caviglia cleared the rubble blocking the Descending Passage.[142]
Opposing the entrance, a blind corridor runs straight south for 11 m (36 ft) and continues with a slight bend another 5.4 m (18 ft), measuring about 0.75 m (2.5 ft) squared. A Greek or Roman character was found on its ceiling with the light of a candle, suggesting that the chamber had indeed been accessible during Classical antiquity.[143]
In the middle of the eastern half is alarge hole called a Pit Shaft or Perring's Shaft. 