The Euljiro underground passageway extends 1.7km from Seoul City Hall, east to Dongdaemuen Culture Park, an interstitial space between the subway tracks below and the urban street above. A byproduct space of the subway cut--‐and--‐cover construction process, the underpass presents a unique type of urban space within the city fabric. Faced with the challenge of channeling environmental elements underground, light, air, etc., this project explores the potential to recycle and reinvent the underground space through the orchestration of the filtering of explores the potential to recycle and reinvent the underground space through the orchestration of the filtering of these elements from above and below, utilizing new innovations and technologies previously unavailable. The new underground passage becomes sensitive to a filter of the urban and transient conditions above and below, creating new environmental synergies between the overlay of different artificial and naturally occurring paderns and frequencies. Solar collectors, fiber optic cables, and light tubes transmit natural light from above while train--‐induced wind from the subway below drives the ventilation for the underground passageway. The result is a new overlap of environmental phenomena, creating a new artificially contrived environmental space within the urban fabric.