The City of Lights sits atop the world's largest maze -- home to mysterious vagabonds, renegade artists, and entire chambers filled with human bones. The Paris Underground, often referred to as 'the catacombs', has been luring curious visitors for centuries. It is cited as one of the world centers of adventure and intrigue.
Let's make it more 'cool' than it is even now. It must remain realistic, but by the time we are done with this, it should be one of the most interesting, adventurous areas in the world, where urban explorers come from all around to delve into its secrets, where the 'catacombs' are much more expansive, holding entire societies.
How, historically, can we do this? Another facet of this challenge: Let's try to change as little of history as we can. We shouldn't be barred from it, but let's do our best.
How does it sound? Nothing from pop culture, either. Cthulu and D&D doesn't belong here.
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My idea begins in the 12th century, when projects such as the Notre Dame and the Louvre were well on their way. An architect receives the King's consent to build a new structure, a huge tower to rival that of the Tower of Pisa. Much more stone is dragged to the surface from these areas. Polished up, the artist is ready to get started, but he is a boorish fellow that comes out of the favor of the King. His confidantes cause something of a civil disturbance. On a whim, the King does the architect an ultimate diservice by dragging his rock cart by cart to the sea, and having the man jailed by the sea to see all of his life's possessions tossed off a cliff.
The artist is executed. The King doesn't want anyone to know, and the legend is lost. Things continue mostly on the level of OTL. Only much more stone is missing from the catacombs.
Continue! Make it big, make it complex, and fill it with people, strange locations, and lore!