The museum I went today, Hamberger Bahnhof - Museum of Contemporary Art in Berlin, which is a former train station.
The exhibition was "Berlin Circle" by Richard Long.

 I liked this huge mud circle which the artist created by hands on site, using mud from the river Avon from his home town Bristol .

A circle, a line: they look good, they are abstract, they are common knowledge.
They belong to everyone and equally to the past, the present and the future.

Richard Long,1982

Here's a video about this exhibition. I cannot understand German, but still interesting to know visually how the exhibition was made.