Often desolate, in isolated fields and overgrown forests, many sites of Second World War mass-shootings of Jews are in danger of being forgotten.
And yet locals remember what happened in these places, and can point the way. Their memories have formed the basis for a new mission to secure and preserve such sites. Of the six million Jews killed, more than a million were murdered by mass-killing units, mostly on the outskirts of towns and cities.
"A lot of the mass-graves have been ignored and fallen into the wrong hands. At some sites, people are digging them up," said Joe Shik, 30, a rabbinical supervisor for the London-based Committee for the Preservation of Jewish Cemeteries in Europe.
Mr Shik recently took part in a pilot project to assess the state of several sites in Ukraine, co-ordinated by the American Jewish Committee, the Central Council of Jews in Germany, and the German War Graves Commission. German's Foreign Ministry has given 300,000 euros to the project.
"It's not like in Germany, where you have Jewish communities around," said Jan Falbusch, program manager at the American Jewish Committee office in Berlin, who led the recent delegation to Ukraine. "There is nothing. You are walking through empty streets which used to be Jewish. You see Jewish traces. You walk in an old Jewish cemetery that is vandalised. So in a way, you kind of experience what the term genocide really means."
The project was inspired by the work of the French Catholic Priest Patrick Desbois, who since 2001 has visited sites of mass-shootings of Jews in Ukraine and collected eyewitness testimonies. Based on these testimonies, which are preserved in Desbois's Paris-based Yahad-in-Unum archive and at the US Holocaust Memorial Museum in Washington DC, the delegation was able to locate and assess the condition of a handful of sites in December.
Few people are left who could point the way to such sites, said William Mengebier of Yahad-in-Unum. Some elderly Ukrainians may not recall dates, but they vividly recall the massacres and often lead researchers to the sites.
Some need to be marked off with fences, and in some cases, another layer of earth may have to be added to cover exposed graves, Mr Shik said.