Abstract: A sealed underground containment vault has an outer shell made of non-biodegradable plastic such as polyurethane or polypropylene. The vault has a containment compartment defined by a bottom wall, and sidewalls and endwalls extending upwardly from the bottom wall. A container, such as a casket, is placed within the lower containment compartment. The outer shell for the lower containment compartment has an upper edge portion that extends completely around the upper edge of the lower compartment. A fusible element, such as an outwardly extending plastic flange, is provided around the upper edge portion completely around the lower containment compartment. A cover for the vault has an outer shell that includes a lower edge portion, which likewise has a fusible element extending completely around the lower edge portion of the cover.