Made out of Elemental Chaos and stretching far "beneath" the Prime Material Plane and "over" the Outlands, this realm of evil has no concrete shape or paths between each of its worlds.
Because of its seperation from the Divine Realms, which are referred to as the Astral Sea, not many beings know that the space containing the lower realms is also part of the Astral Sea.
To a mortal, it simply appears as a nebula of red clouds and angry thunder. Empty evil space with various realms floating within them. Any being shunted into the Elemental Chaos around each Realm within this one will probably immediately die. But if they didn't, they would "fall upwards" towards the Outlands (and most likely die from this too).