What type of burial vault is closed by lowering the dome over the base?

The answer is air seal vault. An air seal vault is designed with a dome that is lowered over the base to create an airtight seal. This design serves a critical function in protecting the casket within, as it prevents water, soil, and other environmental factors from entering the space around the casket. The air seal mechanism ensures that any air and moisture are effectively trapped, contributing to the preservation of the casket and its contents.

In contrast, the other types of vaults mentioned may not incorporate this specific mechanism of closure. Traditional vaults typically refer to a generic burial vault without emphasizing an air-tight feature, while concrete vaults, though sturdy, may not specifically use the dome-over-base design. Steel vaults are also constructed for durability but do not necessarily utilize the same sealing method as the air seal vault. Thus, the distinct method of closure and sealing provided by the air seal vault is what makes it the correct answer in this context.