As part of a public art project, German painter Gunter Demnig installed his first Stolperstein (literally “stumbling block”) in 1995 in Cologne, Germany. Over the past 20 years, the small, cobblestone-shaped memorials have grown into what’s been called the largest decentralized memorial in the world, honoring the victims of the Nazi regime, both survivors and those who perished.

A Stolperstein is typically installed at the threshold of the victim’s last residence or place of employment before being imprisoned, deported, or shot. Per an article published by The New Yorker earlier this year, there are now more than 6,000 Stolpersteine in Berlin and more than 50,000 across Europe.

The one photographed below honors Ernst Ursell who was deported from Kreutzigerstraße 23 on January 20, 1944 and murdered at Auschwitz in Poland.