A burial vault is a structural underground tomb.
It is a stone or brick-lined underground space or 'burial' chamber for the interment of a dead body or bodies. They were originally and are still often vaulted and usually have stone slab entrances. They are often privately owned and used for specific family or other groups, but usually stand beneath a public religious building, such as a church, or in a churchyard or cemetery. A crypt may be used as a burial vault.
References
Burial vault (tomb) Wikipedia(Text) CC BY-SA