A gas explosion followed by fire outbreak damaged surface installations at an underground aquifer natural gas storage centre. The accident occurred in an electrical utility room, outside the gas supply zone, containing the programmable controllers of a transfer manifold, while the storage was in a drawdown phase. Both the emergency shut-off switches and alarm were activated. The site set up a safety perimeter and suspended all activity. Employees wearing self-breathing apparatuses quickly controlled the accident using CO2 extinguishers. Only equipment damage was recorded: a cracked building, cabling and control-command boxes to be verified, and the cast iron lid of a drawing chamber was ejected several m. A gas leak was ultimately detected on an idle arm pipe (NPS50) connected to an NPS150 “branch” on a bleed circuit; the leaky pipe had been partially disassembled since 1987. The path between the leak point and the room of the accident likely followed a series of ducts housing the electrical cables. The leak, whose duration could not be precisely determined, might have started before the accident. The pipe buried 3 m deep was isolated and drained. In overcoming the difficulty of 2.5 m of ground frozen by pressurised gas, a pit was finally completed 2 days later. The leak volume amounted to between 2500 and 25,000 m³. Inspection authorities suggested a Prefectural order subjecting the resumption of storage operations to submission of thorough accident report, accompanied by accounts of site repair works. The operator was also requested to subsequently establish an action plan for treating vulnerable points along the pipeline like the one observed during the accident, in addition to upgrading gas detection on premises not yet equipped and in the ducts. The operator replaced the identified pipe section and removed the idle arm. The collection at other well sites was also assessed: elimination of the idle arms on former piping, and a corrosion diagnostic for the other pipes. Measures intended to improve the seal on electrical premises had begun implementation. These studies led to the installation of a servo-control between gas detection and the power supply shut-off switch. The pipe leak might have been due to metallurgical defects; additional analyses were conducted.