underground

to pick up on julian's explorations, and michael's follow-up, an example of underground work...
http://www.archdaily.com/43187/villa-vals-search-cma/



and 2 seminal examples of underground habitats... or work by substraction... Tungkwan [China] and St. George Church at Lalibela [Ethiopia]
http://www.theatlantic.com/entertainment/archive/2012/01/the-collaborative-design-of-ancient-hive-cities/251898/