The Depths of Unknown was a stratum of Skaro located above the Abyss of Death and below the Stratum of Screaming Winds, being the fourth stratum from the surface. The Fissure passed through it.
It was home to the Krakis, who were its masters. Due to the Krakis, this was "perhaps the most terrifying area in Skaro's depths". (COMIC: The Dalek Trap [+]Loading...["The Dalek Trap (comic story)"])