Holocaust mass grave sites in eastern Europe can be difficult to investigate due to a paucity of historical documentation relating to the events and because the use of traditional invasive archaeology methods raises concerns around the disturbance of the remains of Jewish people. When combined with other lines of evidence, including historic photos and eyewitness testimony, noninvasive geophysical methods help to effectively identify and demarcate buried features at Holocaust sites, limiting unnecessary excavations. Between 1941 and 1944, as many as 100,000 people were murdered at the Ponary (Paneriai) extermination site in Lithuania, but many critical details of the site layout during this period are still to be resolved, including the location of some of the mass graves and confirmation of an escape tunnel that was used by slave laborers to escape captivity and certain death at the site. At Ponary, a combination of electrical resistivity tomography (ERT) profiling, limited ground-penetrating radar data, and bare-earth elevation data (from a light and distance ranging data set) was used to confirm the location of a large unmarked mass grave with a diameter of approximately 25 m and depth of approximately 4 m. Additional ERT profiling at a second location imaged the entrance to an escape tunnel previously uncovered by an archaeological excavation in 2004, and it detected an approximately 5 m section of the continuation of the tunnel, approximately 33 m away from the tunnel entrance. The geophysical results are supported by evidence from limited archaeological excavations, historical photographs, eyewitness descriptions of the site layout, and testimonies from the few survivors who managed to escape Ponary.

A common practice used by the Germans and collaborators in World War II, as part of the Holocaust, was to use existing Jewish cemeteries as places for mass burial. Research was completed at the Old Jewish Cemetery in Riga, Latvia, the Livas Jewish Cemetery in Liepaja, Latvia, and the Zaliakalnis Jewish Cemetery in Kaunas, Lithuania. The Old Jewish Cemetery in Riga was adjacent to the Riga Ghetto and was used to bury individuals murdered in the ghetto. In Kaunas, an area of the Zaliakalnis Jewish Cemetery is devoid of grave stones, and literature sources and testimony indicate that this area was used for the mass burial of Jews from the Kaunas Ghetto and other mass killings. In Liepaja, the local Jewish Heritage Foundation believes that there are mass graves within the Livas Cemetery. Methodologies for this research include the use of a pulseEkko Pro 500-megahertz ground-penetrating radar (GPR) system. Electrical resistivity tomography (ERT) data were collected through a linear array of electrodes coupled to a direct current (DC) resistivity transmitter and receiver. Analysis of aerial photography and satellite images was also employed at each location. ERT and GPR data indicate three separate trench anomalies in the Old Jewish Cemetery in Riga. The presence of these anomalies corroborates Holocaust survivor testimony that bodies were buried in mass graves in that area. In the Zaliakalnis Jewish Cemetery in Kaunas, ERT and GPR data indicate an anomaly in the western part of the cemetery, and ERT data further indicate two other possible mass graves. In Liepaja, preliminary GPR analysis indicates an anomaly in a cleared section of the cemetery. Based on the presence of geophysical anomalies in all three cemeteries, which correlate with literature sources and Holocaust survivor testimony, there is a high probability that mass graves are present at each site. Future research directions include expanding the search areas in each cemetery, additional literature and testimony-based research, and the addition of other geophysical methodologies.

Archaeological site investigations in urban environments are often beset with challenges such as (1) an absence of buried artifacts due to recent disturbance from infrastructure development or (2) community concerns about potential site impacts from excavations. Noninvasive geophysical surveys that use a combination of methods can help mitigate the risks of uncertain outcomes by identifying areas where culturally significant features are more likely to be uncovered. We show how new technology and traditional geophysical survey methods were used to characterize the subsurface of the Mila 18 Memorial site in Warsaw, Poland. This site is one of the most important places of remembrance for the Holocaust and coincides with the location of an underground bunker that was used by Jewish resistance groups during the 1943 Warsaw Ghetto Uprising. In this case study, we showcase the use of drone multispectral imaging and handheld lidar scanning in conjunction with other geophysical techniques including electrical resistivity tomography, ground-penetrating radar, magnetic gradiometer, twin-probe resistance, and fixed-frequency electromagnetic surveying. The geophysical results were included in an interactive 3D site model to help identify a suitable site for excavation. To document the excavation and to validate and further interrogate the geophysical survey results, we used lidar-based photo-textured scans of the excavation that were incorporated into the 3D site model.

Jacob Gens, the head of the Vilnius Ghetto Police Force, and eventually the entire Ghetto during the Holocaust, was murdered on 14 September 1943 by the head of the Vilnius Gestapo. Historical documents and Holocaust survivor testimonies indicate that he was killed at a site that became known as the Rasu Street Prison, and not the Gestapo Headquarters, as it is widely believed. In 2016, research was completed at the Rasu Street Prison site using ground penetrating radar (GPR) and electrical resistivity tomography (ERT) to locate subsurface reflection patterns that possibly indicate the location of where Jacob Gens is buried. Intersecting GPR and ERT reflection patterns were discovered and a plan was put in place to excavate that location. The excavation revealed the presence of human remains at 1.45 m below the surface. A skull and upper torso were exposed, and two teeth were collected for DNA and radiocarbon analysis. The DNA from the tooth was compared to Jacob Gens’ daughter’s DNA, and this comparison yielded a negative result, so the human remains were not those of Jacob Gens. The radiocarbon analysis provided a date between 1685 and 1735. In 1705, a plot of land was donated to the Visitation Monastery, which used this plot, and which coincides with the location of the Rasu Street Prison, as a cemetery for the poor. In 1709 and 1710, a plague epidemic was prevalent in Vilnius, as was turmoil and famine associated with the Great Northern War (1700 to 1721). Based on these discoveries, rather than finding the remains of Jacob Gens, it is likely that we found human remains that are part of a forgotten 18th century cemetery associated with the Visitation Monastery.

Excel file of the data (Magnetic susceptibility and faunal record) from sedimentary cores. The data are free to use for everyone interested.