No one was supposed to find Sobibor ever again. In 1943, when the Nazis shut down the death camp, they destroyed the gas chambers. To cover up pits where they buried their victims’ ashes, they planted trees.

But their attempt to totally erase the camp failed. The marshy ground in an Eastern Polish river valley still held the victims’ last possessions, rusty and buried beneath the soil. From 2007 on, archaeologists dug up tens of thousands of objects on the former camp terrain. Traces in ...