The Zeitgeschichliches Forum - the Forum of Contemporary History

A display of locked lids in East Berlin to prevent escape through the sewers.

The sign next to the lid says:
Cordoned Off
After 1945, Berlin is divided into four sectors.
Signs in West Berlin indicate the borders: the languages point to the city’s special status.
The first border wall is built of the hollow concrete blocks often used for the housing in the GDR.
V-brackets with barbed wire are placed on them.
The barriers also have concrete posts.
Lockable manhole covers prevent escape.
Barricades of welded railroad tracks are designed to keep cars from breaching the border posts. Click for a larger image