An improved underground tank assembly is provided for storing motor fuels, such as gasoline or diesel fuel. the underground tank assembly has an expandable bladder-like primary tank positioned within the interior of a rigid secondary tank. Advantageously, the secondary tank meets most safety regulations and secondary containment laws by dependably providing for secondary containment of any leakage of motor fuel from the primary tank. The underground tank assembly has numerous alternative conduit arrangements for ease and flexibility of assembly, installation, and manufacture. The underground tank assembly also desirably has a protective barrier wall for protecting and isolating the conduits from the expanded primary tank. Other safety equipment and controls, such as leakage detectors, are provided.