5. Conclusion

Like Monsieur Jourdain's prose, underground urbanism was practiced long before it was recognized, named and conceptualized.

If, in ancient times, Babylon, Alexandria and Rome were exceptions in the midst of a rural society, today's society is "massively urban", according to Françoise Choay , who adds "... it is struggling to develop the city...". The same author seems to regret the excesses of a regulation that she recognizes, however, as indispensable.

In any growing city, underground space is a safety valve against congestion. Although it is widely available and rich in possibilities, the subsoil is ignored or, to say the least, underestimated,...