The area today named the Underground Chamber now looks very different from how the Prince of Sansevero originally envisaged it. He conceived it as a small funerary temple for his descendants, half above and half beneath the level of the main nave of the Chapel. For reasons unknown, however, the project was never finished by Raimondo di Sangro or his heirs.
In a few letters addressed to the Academician of the Crusca, Giovanni Giraldi, and to the physicist Jean-Antoine Nollet, the Prince describes his design in detail. The oval temple (the shape is still preserved) “will appear to be excavated from the rock, and will be well lit by a cupola, which shall have a number of windows”. It would have been “divided into eight arcades with the same number of columns”, and the arches would have housed the marble caskets of the bones of his descendants.
Above all, the centre of the underground temple would have housed Sanmartino’s Veiled Christ, lit by two eternal lamps designed by the Prince and placed at the feet and head of the statue. It is not known with absolute certainty whether the Christ was ever placed there, but it is highly improbable. A slab of marble placed at the centre of the Chamber shows the intention of di Sangro to place the Neapolitan sculptor’s masterpiece here.
This cave tomb (the Prince described it as “une éspèce de caveau”), illuminated by a perpetual flame and enriched by the beauty of the Veiled Christ, was meant to have, within the overall design of the Chapel, a symbolic role which was far from secondary for its designer. Currently, the Chamber houses display cases with the two Anatomical Machines and a large fragment of the eighteenth-century flooring.