During his round, the security guard of an oil depot noticed a leak close to the manhole of tank 121 containing 32,000 m³ of crude oil with a flash point below 40°C. He drafted an equipment anomaly report. 10 m³ of hydrocarbons (HC) spilled out over the 150 m² soil retention system in a layer a few cm thick. Several leak points were visible around the entire periphery of the bottom of the tank. The leak rate was estimated at 1 m³/h. The operator stopped acceptance operations throughout the site, installed a foam gun (6,000 l/min) in the vicinity of the retention system, shut down the tank’s agitators, installed 2 explosive vapour and hydrogen sulphide detectors, then drained the tank at a flow rate of 1,000 m³/h towards the refinery and another of the depot’s tanks, thus reducing the leak rate to 200-300 l/h. A tank of foam concentrate was transported from the refinery where 2 security service officers monitored the development of the leak. Atmospheric measurements were taken every hour. The piezometric analyses carried out in the water table (over 10 m deep) were unsuitable because the 2 measurement points were dried out. A land surveyor verified the tank’s stability. An external company installed props for the floating roof (but did not comply with the recommended air control safety measures). A pumping well was created to recover the HC, the pool of oil was channelled to 2 low points using sand bags, and absorbent was spread where the HC could not be otherwise collected. The SDIS (French departmental fire service), called at 5 pm, took gas explosion measurements. On 11/11, the tank was drained and contained only a 20 – 25 cm layer that could not be recovered as it contained 500 to 600 t of sediments. On 12/11, the seepage observed on the surface of the tank stopped. By 13/11, 15 m³ of crude oil had been pumped into the retention system, and 20 to 25 m³ of HC were estimated to have been lost. The inspection authorities for classified facilities noted on that same day that the operator had not yet, as planned, installed plastic film between the bottom of the tank and the patrol path to prevent rainwater from seeping into the polluted soil. The operator planned to carry out coring at the bottom of the system and remove gas from the tank after the drainage in order to work on it. It supplied the inspection authorities for classified facilities with an accident report. It excavated 120 m³ of polluted soil. Corrosion may have caused the accident. Indeed, the stored product contained salt water and during previous visits, the inspection authorities for classified facilities had noticed that some of the agitators used for preventing this phenomenon were faulty. In addition, contrary to the monitored equipment inspection procedures recommended by the parent company, the operator had only checked the sheet metal at the bottom of the tank during the ten-yearly inspections, and prior to 2006, these checks had been delayed twice with no compensatory technical provisions.