When hungary opened it's borders, it created a "hole" in the blank.

When Hungary opened its borders, it created a "hole" in the Iron Curtain. Prior to the opening of its borders, the Soviet Union had largely prevented the crossing from the Soviet Bloc into Western Europe through the construction of fences and walls. However, once Hungary opened its borders with Austria, many living in Soviet-controlled nations fled through this gap, an act which led to future events such as the Fall of the Berlin Wall.

Nm-us history and geography