Abstract: An underground tank storage (UTS) system includes one or more storage tanks disposed within a vault. The vault may comprise a plurality of concrete sections arranged in mirror image about a horizontal mid-plane. Center sections are U-shaped, having a horizontal panel and end walls, and are arranged in upright and inverted pairs. End sections are also U-shaped with an additional wall extending between the legs of the section. These likewise are arranged in upright and inverted pairs. The vault sections are sealingly secured together with grout keys and joint tape. A suitable material resistant coating is disposed on the lower interior surface. An inert gas atmosphere is maintained within the vault. The inert gas provides an oxygen and moisture free atmosphere which improves the service life of the tanks, enhances safety and leak detection capability and inhibits the likelihood of combustion. Vapor or liquid leak sensors are provided to sense and respond to a tank leak which develops in the vault.