The Schwerbelastungskörper is a massive concrete cylinder built in 1941 to test whether Berlin's sandy ground could support Albert Speer's planned monumental architecture for Nazi Germania. The answer was effectively no, and the test block was too heavy to remove easily after the war.
It is one of Berlin's stranger surviving Third Reich traces: blunt, ugly, and useful precisely because it shows the scale and absurdity of the plans. A small information area explains the context.