The underground heritage located under Paris and its suburbs is threatened with disappearance in the short and medium term due to urbanisation. Less than 1% of the surface of underground quarries is protected. The gradual disappearance of these sites is done in a total indifference of the public authorities and even sites protected by law are threatened with at least partial disappearance. It is the case today in Meudon (Hauts-de-Seine department) where the underground quarry “Les Brillants”, which is a protected area since 1986, is currently under threat of being filled in. Due to the approximate control of the underground risk by the urban developers, half of the protected underground quarry of Meudon is threatened with destruction. The filling of underground quarries located in urban or suburban areas is systematically chosen for technical ease, financial interests and lack of awareness of the heritage value while other solutions could be envisaged to reinforce these places if it were essential, while safeguarding them. When backfilling is necessary, as in the case of limestone or gypsum underground quarries with major collapses, no assessment of the natural or cultural heritage objects is carried out before the filling.