ABSTRACTThe Great Tumulus of Apollonia in northern Greece, with a diameter of ~100 m and a height of 19 m, is among the largest of its kind in the region of the ancient kingdom of Macedonia. It is located north of the ancient city of Apollonia and recently became the focus of limited archaeological excavations, which revealed a looted Macedonian tomb. Archaeological findings and other evidence from the tumulus and its surroundings suggest that it may have been used more than once; therefore, the existence of more tombs in its interior is possible. In this work, we investigate the internal structure of the monument by means of 3‐D seismic travel time tomography. Using direct sparse methods, we calculate efficiently the full model resolution matrix that allows us to investigate the robustness of the tomographic model. Our results suggest a complex structure with variable properties between the east and the western side of the tumulus. We also detect several regions that may be associated with additional burial locations or other possible targets of archaeological interest.