Although over half of mankind lives in an urban environment, the space available underground remains an asset little used so far other than for municipal engineering purposes and isolated projects. Despite the high density of buildings that saturate the earth’s surface at present, reflections on town-planning very rarely consider underground areas as being some living space. Exploited by man for several thousand years, notably for extracting materials or as shelter, underground areas have been used by city-dwellers for many varied activities since the first half of the 20th century. As a result, above- and below-ground urban synergies have been created in different metropolosises, thus revealing all the interest in exploiting underground space for city-dwellers’ quality of life both above and under the ground.