As part of the extension of line E of the Regional Express Rail Network of Paris, it was decided to create an underground station under the CNIT (National Center for Industries and Techniques) located in the business district of La Défense. This project clearly raises the question of the stability of the main vault of the CNIT under the effect of any soil movement caused by the proposed works. This vault, which still holds the world record of span length, was built between 1956 and 1958 on a project of Nicolas Esquillan. The article briefly describes the history of the vault construction, and presents the results of finite element modelling and geotechnical calculations, the results of the destructive and non-destructive tests conducted on the vault, the control of the geometry and reinforcement, the few observed disorders, the condition of the ties and their tension. It draws conclusions on the general condition of the structure from the point of view of its strength and durability. The quality of the work carried out in 1958, both on site and in the design office, the attention paid to the materials chosen and the work processes on site, make this structure age very well