Horizontal cylindrical single and double-walled tanks for underground storage of flammable and non-flammable liquids. Made by using shell plates and dished ends made in carbon steel quality S 235 JR complying with UNI EN 10025. The external and internal joints are tight welded. All components are produced according to rules and regulations in force and in particular tanks are designed and manufactured in compliance with the European Standard UNI EN 12285-1. Single-wall tanks: form an impermeable containment; they act as the inner tanks of double-wall tanks. Double-wall tanks: for these tanks, a secondary wall is welded around the inner tank, which forms an impermeable self-contained tank. These tanks are provided with two sockets for the leak detection system in order to permanently monitor the integrity of the tank. All tanks are provided tested. Tests are pressure tests and are performed to check the integrity of both inner tank and outer tank.