The Crossing is a dimensional doorway between two different versions of Earth. The portal is at the heart of the new Starz Original Series Counterpart.
The process of passing through The Crossing involves a search of one’s person and belongings by personnel in military dress. There is some sort of blue-light scan of the hands as well. The customs crossing room appears to be in the basement, the walls are thick slabs of stone and the area is partitioned with fencing and sliding metal doors. Each individual is required to cross alone.
Persons walk through the sliding metal door and wait in a circle while a larger, thicker metal door cycles through what appears to be an airlock sequence. Once the light above the door goes from red to green, the door opens and the person is permitted through.
The door leads down a short flight of stairs, across a dark stone room and back up an identical set of stairs on the other side. In this room, there is the sound of tectonic plates shifting and unseen machines buzzing. Once up the stairs on the other side, the entire process then repeats in reverse with the door cycling open on the holding area with the sliding metal door leading to a fenced-in area and the customs crossing office.
In 1986, during the Cold War between the United States and the USSR, there was an accident during an experiment in the USSR-controlled area of East Berlin. At the time, the universe split into two identical copies (Dimension One and Dimension Two).
While identical in every way at the moment the Crossing opened and for several years after, the two sides hit The Break and diverged significantly after that.
The location of the accident, now known simply as "The Crossing," became a nexus point allowing travel between the two dimensions
The Office of Interchange is the name of the bureaucracy that controls access, diplomacy, and espionage between dimensions.