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.
Thirty years ago, 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. The site of the accident, the area now known simply as "The Crossing," became a nexus point allowing travel between two dimensions.
While identical in every way at the moment the Crossing opened, the two sides have diverged significantly in the 30 years since and are now involved in a cross-dimensional cold war with each other.
Bureaucracies on both sides of the portal control access, diplomacy, and espionage between dimensions through the Office of Interchange.