Negative Zone (Earth-239)

Sub-space dimension partially explored by the Fantastic Four.

History
Discovered by Reed Richards, the Negative Zone is a dimensional space where all matter features a negative charge. Prolonged exposure to the Negative Zone can have deleterious effects on both human psychology and biology, although they have yet to be fully discovered.

Nick Fury's Notes
Personally I can't wrap my head around all this "alternate dimensional planes of existence" stuff. One of the eggheads told me that the Negative Zone was "less of a place, and more of a state of being," but it looks like a place to me. A damned unpleasant one at that.