Impenetrable Rock

The Impenetrable Rock, also known as Solid Rock in Dungeon Keeper 2, is simply impassable terrain that cannot be dug through at all, although there are certain campaign levels of Dungeon Keeper 2 in which there is a secret way to remove a particular section of it to release a trapped creature.
It is often used as the 'boundaries' for each map, surrounding the edges of the level. It is also often used in campaigns as either a hint (especially in the earlier levels of Dungeon Keeper, where some rooms are surrounded by the rock in an unnatural square-shape.), or as a way of making the game more challenging, such as forcing the player to find another way around.
An interesting note is that in Dungeon Keeper, the location of Impenetrable Rock is always visible, unlike in Dungeon Keeper 2 where is it still hidden by the Fog of War until discovered.