Just a concept mock up I drew for the Chronicles of the Infinite Realms. The Cold Void is described as follows, right out of the PHB I wrote:

Conversely going deep into the earth will not find the Amaranthine but the earthen wall equivalent: The Cold Void.

The Cold Void is a cavernous abyss that stretches into oblivion beneath all Realms. Should one dig deep enough they will find the Cold Void. A truly bottomless hole where the chill air can cool even the molten magma of subterra to stone. It is said that the Cold Void links all Realms in the same way that the Amaranthine does, but likewise it is just as perilous to explore, if not worse. For dark things lurk in the depths of the Cold Void, things that have not seen light in countless millennia, since the early days when the Realms were young. Mining expeditions and even the brave badgers do their best to not breach the abyss. To make things further complex, the depth of the earth varies from place to place, Realm to Realm, ultimately it’s just best to stay above the magma layer.