Missy points to a spherical device situated in the middle of the area and names that as the Nethersphere, a Gallifreyan memory device, where the memories of the dead are stored and held to be inserted into new Cybermen soldiers after having their emotions deleted.
Missy points to a spherical device situated in the middle of the area and names that as the Nethersphere, revealed to be a Gallifreyan memory storage system known Matrix. Missy explains that the consciousness of the deceased are stored and held to be inserted into new Cybermen soldiers after having their emotions deleted.