Library of Alexandria

The underwater library used as the setting and the main topic of volumes 4 and 5.
As explained in-story, this is not the publicly known Library of Alexandria, but instead, a building constructed by the combined efforts of a branch of the Atlas Institute during Ptolemy I’s time. In other words, a Lost Number that even the Atlas Institute itself completely forgot about.
Consequently, research results believed to be recorded only in the Pseudo-Spiritron Computer TRIHERMES had been left untouched in the Library of Alexandria.
The story reveals its true roots as the product of a tangled web of secret plots by several groups and individuals, but regardless, its ruins are overwhelmingly precious and a rare find.

TYPE-MOON BOOKS material: The Adventures of Lord El-Melloi II Glossary