The Infinite Featureless Plane of Death is the realm of death in Irregular Webcomic. The plane is actually one big grey LEGO baseplate. Despite the name, the plane does have one feature; the Head Death's desk. It may also not be infinite, as Death of Inhaling Hatmaking Chemicals once told Jamie that the plane had a slight positive non-Euclidean curvature, ultimately making it a finite plane with one feature. However, he may have been mistaken.
On their second 'visit' to the Infinite Featureless Plane of Death, the Mythbusters blew a large hole in it, leading into the Underworld. Whether this hole has been discovered yet, or even whether it will be permanent, remains to be seen.