This article presents the results of a ground penetrating radar (GPR) and Electrical Resistivity Tomography (ERT) investigation carried out in the atrium of the Cathedral of Puebla to shed new light on the location of the original temple known as the "Old Cathedral". Historians who endeavor to reconstruct the past of this historical monument propose two hypotheses. To verify whether the foundations are located on the northwest or northeast side of the current cathedral, a GPR study was conducted with 200 and 400 MHz antennas. The study was completed with an ERT survey. The remains of the old cathedral were located and identified in the northwestern part of the atrium. Further anomalies related to subsurface structures and consistent with the ephemeral existence of a smaller temple, the "Sagrario", were detected. The GPR proved to be the most suitable method to investigate without causing damage suspected subsurface remains, thus protecting cultural heritage.