Saw this Stolperstein, or "stumbling stones," which are bricks that are installed in the cobblestone sidewalks in European cities to commemorate the victims of the Holocaust. They are slightly higher than the other stones to make them more noticeable to people walking over them.
Over 75,000 stolperstein have been laid in 18 European countries. The project was was initiated by the German artist Gunter Demnig in 1992