An underground passage is a pathway that runs below the ground surface, commonly used to travel or transport goods discreetly. There are several synonyms for the term, including subway, tunnel, underpass, and crypt. These words are often utilized to describe the same sort of infrastructure, though subway exclusively refers to a system for public transportation, and tunnel typically implies a structure with a reasonably significant length. Underpass tends to be a term used for vehicular or pedestrian access, while crypt emphasizes the idea of secrecy and hiddenness. No matter the term, underground passages provide an efficient means of transport that can be used in different contexts.