Although few people pay attention to the architecture beneath the Louvre Museum, detailed records of it still exist in history, along with some technology channel programs that have explored the underground palace, all of which provided Yan Fei with material that helped him find enough useful information. After the collection and organization by the Auxiliary Brain, he was able to deduce a complete 3D Map of these underground structures with the scant information he had.
The abandoned passage Yan Fei found was leading to the underground buildings of the Louvre Museum; it was the museum's earliest drainage pipe, which was abandoned after the Louvre had its drainage system redesigned and updated.