https://en.wikipedia.org/wiki/D%C3%BCnam%C3%BCnde_Action#The_D.C3.BCnam.C3.BCnde_actions

In the western Ukraine, SS General Friedrich Jeckeln notices that the haphazard arrangement of the corpses meant an inefficient use of burial space. More graves would have to be dug than absolutely necessary. Jeckeln solved the problem. He told a colleague at one of the Ukrainian killing sites, 'Today we'll stack them like sardines.' Jeckeln called his solution Sardinenpackung (sardine packing). When this method was employed, the victims climbed into the grave and lay down on the bottom. Cross fire from above dispatched them. Then another batch of victims was ordered into the grave, positioning themselves on top of the corpses in a head-to-foot configuration. They too were killed by cross-fire from above. The procedure continued until the grave was full."