@ossobuco,
The Grenzübergangsstelle Marienborn, or border crossing Marienborn was the largest and most important border crossing on the inner German border during the division of Germany.
Between 1972 and 1974, the GDR erected a new control point on a 35 ha field situated on a hill near Marienborn, about 1.5 km east of the border. The control point was staffed with up to 1,000 passport control, customs and border police employees. The buildings were linked with an underground tunnel system through which military or police units could reach the control point quickly and in secrecy.