Early in the morning of 24 August, 2013, following by hours the drilling of a shallow borehole in the same spot, a new fumarole producing emissions of CO2-rich gas, water, and mud suddenly appeared at a crossroad along the fenced area of the Fiumicino international airport of Rome, Italy. Similar episodes have been scientifically documented or simply reported in recent and past years. To understand why gases are easily entrapped in the shallow subsurface of the Fiumicino area, we used five borehole cores drilled by us, analyzed the stratigraphy of these and other nearby cores, acquired a 2D seismic refraction tomogram, and performed chemical and isotopic analyses of water samples collected from aquifers intercepted by two drilled boreholes. Our boreholes were realized with proper anti-gas measures as, while drilling, we recorded the presence of pressurized gases at a specific permeable gravel level. Results show that, in the study area, gases become mainly entrapped in a mid-Pleistocene gravel horizon at about 40-50 m depth. This horizon contains a confined aquifer that stores the endogenous upwelling gases. The gravel is interposed between two silty-clayey units. The lower unit, very hard and overconsolidated, is affected by fractures that allow ascending gases to bypass the otherwise impermeable shale, permeate the gravel, and dissolve into the aquifer. In contrast, the upper unit is impermeable to fluids and seals the gas-pressurized aquifer, which therefore constitutes a source of hazard during human activities such as well drilling, quarrying, and various building-related excavations. As the stratigraphy of the Fiumicino area is very common in large portions of the densely populated Roman area and as the adjacent volcanic districts are hydrothermally active, we conclude that phenomena similar to that observed at Fiumicino could again occur both at Fiumicino and elsewhere in the surrounding region. As a prompt confirmation of our conclusion, we signal that, while writing this paper, new artificially triggered degassing phenomena occurred off Fiumicino in connection with the construction of the new harbor.