Zimmer’s team members—who worked in hazmat suits to protect themselves from the lead-rich yellowish deposits that coated the cathedral after the fire—also recovered 1000 pallets of stone and metal items. It was largely impossible to recycle these salvaged materials for restoration work, as they had been damaged or weakened by the intense fire. But the debris “played a central role in advancing our understanding of Notre Dame’s structure and informing the restoration,” says De Luca, an architect and computer scientist.

For example, archaeologist Cédric Moulis of the University of Lorraine painstakingly reassembled dozens of wedge-shaped stones called voussoirs recovered from the collapsed vaulted ceiling over the nave to glean insights into their mechanical properties. That allowed architects reconstructing the cathedral to ensure that the new vaults have a similar tolerance to forces as the vaults had before the fire, says Stéphane Morel, a mechanical engineer at the University of Bordeaux.