a ground-level or semi-subterranean vaulted corridor, usually lit by openings in the vault

In Ancient Roman architecture a cryptoporticus (from Latin crypta and porticus) is a covered corridor or passageway. The cryptoporticus is often at ground level or partially underground, and supports a structure such as a forum or Roman villa, in which case it served as basis villae. It is often vaulted and lit by openings in the vault.