An "mind playing tricks on your vision" experience where the viewer thinks he's present at "the other place".
Marty McFly opines that the dystopian wasteland which Biff's corruption and debauchery have created of Hill Valley is like the stompin' grounds of "The Grim Horned One", but Doc Brown assures him that it is indeed just their hometown, but that probably it really isn't that much of a Hellucination, since he cannot imagine that said "down below" locale would be likely to be much worse.
by QuacksO March 24, 2022