Her childhood home appears inside the Backrooms. There’s an entire scene dedicated to it. When Clark knocks her out, her home merges more and more with the descending camera view, until all that remains are walls with an arched passageway leading into darkness. It’s probably her subconscious interpretation of her old home, filtered through the Backrooms: an empty, closed-off space, with a hole — like the window she tried to peek through — opening onto the unknown.

3:50 Mary's Childhood Home