The underground is strongly linked to imaginaries woven by humans' actions towards the unexplored world below their feet. It has nourished myths and fictions about the afterlife, escapism and exile. In Plato's Republic, the allegory of the soldier Er, descending to the Inferno (Richardson, 1926) revives humanity's fears and beliefs about communication with the unknown and the challenge of limits. In Homer's Iliad, the dead also descend to a subterranean realm ruled by gods. In the Homeric Odyssey, the katábasis is described as a heroic journey to a supernatural world (Burgess, 2016). Although underground ways of life have been envisaged in literature, fiction, religion and folk tales, human settlements throughout history have reaffirmed practical ways of living on the Earth's surface through adaptation to each region's climate. Visions and imaginaries about the underworld have profoundly changed with the advent of industrial society, by mechanising, measuring and wishing to conquer every inch of the terra. The underground world was rationalised while it was accessed, tamed and forged by new aspirations about the expandable limits of humankind's territory. The troglodyte (from the Greek, meaning living in holes) houses and settlements dispersed around the globe remind us of human beings' ongoing efforts to protect their bodies from hostile weather, but also of the effort to deal with the scarcity of building materials and comfort of housing conditions. But what link could one make between Friedrich Engels's criticism of the unhealthy, wretched conditions of the working classes in industrial Manchester and Liverpool, the semi-underground apartments for migrant populations in Athens (Maloutas & Botton, 2021) or the abandoned bunkers providing refuge for the unwanted of society's global metropolises? The stratification of classes in human settlements is the result of finite, valuable (for its use) space and the constant quest for better living conditions "on an upper level" with sufficient air and light. This social stratification of the urban environment was celebrated by architects and planners of the 20 th century 1 who wished to structure the city vertically -such as Introduction to the special issue Underground Atmospheres Ambiances, 8 | 2022