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A hazardous gas eruption from two very close shallow boreholes occurred near the Fiumicino International Airport of Roma (Italy) from August to December 2013. The erupted gas was mostly CO 2 of deep origin and gas output was high and sustained over time reaching values of nearly 20 t day − 1 . After 3 months, the gas flux was still above 5 t day − 1 and was only stopped in December 2013 by long and expensive works of closure of the boreholes. The gas eruption was uncommon as being associated with the building of two mud volcanoes. This style of sustained deep CO 2 eruptions contrasts with the more common short-lived eruptions of shallow biogenic methane-dominated gas pockets. In this work, we present the chronology of the event, the results of geological, geochemical, and geophysical monitoring and a numerical modeling. We propose that the August–December 2013 sustained and prolonged event does not relate to the simple degassing of a shallow, isolated pocket of gas. On the contrary, it reflects very specific conditions in a shallow reservoir (hosted in a 10 m thick gravel layer at − 40 m within the Tiber river delta deposits), related to the interplay between the total pressure and the fraction of free CO 2 initially present, across very narrow value ranges around 0.59 MPa and 0.18, respectively. The coexistence of short-lived and long-lived eruptions from the same reservoir suggest that these conditions are not achieved everywhere in the gas reservoir, despite its homogeneous properties. This consideration implies either a pressure compartmentalization of the reservoir, or the occurrence of a transient, possibly associated with an impulsive release of gas from greater depths. The involvement of deeper and larger gas reservoirs connected along faults is evidenced by geophysical investigations. This conceptual model bears significant implications for gas hazard studies. [ABSTRACT FROM AUTHOR]