A contractor at an upper-tier Seveso plant was cleaning an underground tank so that it could be filled with a different product. The tank was located in an outdoor area comprising 12 horizontal underground multi-compartment tanks, each having an overall capacity of 60 m³ (nominal compartment capacities of 10, 15 and 35 m³) and containing flammable liquids (solvents or petroleum products). Cleaning began at 8:30 a.m. At 9:55 a.m., an explosion occurred in a 35 m³ compartment containing ethyl acetate. The force of the explosion sent the tank’s cover as well as tools and debris flying across a distance of several meters. The contractor’s two employees suffered severe burns. The visible property damage was limited to empty pipes used to transfer solvent, which received slight damage from falling objects. The explosion cut off the plant’s electricity supply, in turn shutting off its computer system and the automated systems used to monitor equipment on the underground tanks. The operator administered first aid to the two injured employees (absolute emergency), immediately called emergency services, initiated its internal emergency plan, and notified nearby businesses. A large number of first responders (30 gendarmes, 56 firefighters from the Departmental fire service (SDIS), and 14 vehicles) were quickly mobilized.