A typical approach for karstic carbonate risk assessment is to utilize integrated geophysical research and calibrate the data with either pre- or postmeasurement acquisition borehole information. Here, we present a case study of a time-demanding investigation on the detection of subsurface areas prone to new building foundation stability at a site rich in karstic structures belonging to the Trypali Unit (Triassic-Jurassic). During excavation and construction to expand the airport in Chania, Crete, Greece, a large karstic void was revealed, which triggered an alert that led to an immediate work stoppage and the demand for an investigation of the subsurface of the whole area. The scope was to detect weak zones or voids larger than 0.5 m with a fast and timely approach because the costs of a long project stop are high. To meet the time demands with an efficient approach, we utilized the electrical resistivity tomography method, which guided a fast postacquisition borehole program and was supplemented by in-borehole video recordings, aiding in the direct detection of karstic structures. The 3D inversion of the electrical data provided electrical resistivity tomography images of the subsurface, which characterized the area as highly karstified and fractured and detected voids with sizes ranging from 0.5 to 6 m.