system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are the Paris Catacombs so hard to map.
gpt
First off, they’re dangerous for a number of reasons (the difficulty of navigating/mapping them is just one reason) so there aren’t many professional organizations spending time down there or otherwise devoting major resources to exploring them. I’m sure there are plenty of people who explore them on their own time but they aren’t part of a well organized or regulated mapping effort. 

Because they’re underground and tunnels could be at different depths, you often wouldn’t have a reference or hint at your location when different tunnels cross each other— like city streets must do because they’re all at the same elevation.

Compasses work down there so you can track direction but distance is tricky, especially if you’re counting distance but your direction is constantly changing.

GPS definitely does not work underground.

You also can’t use an aerial photograph or a number of other visual techniques that you’d use above ground.

Actually, it’s super hard to map underground tunnels *anywhere*, and renovations of subways and sewers in other cities rely heavily on records of where existing tunnels were built. And they can run into serious problems when those records are incorrect or missing.

So when you have a large, complex system of tunnels like the Paris catacombs, and it’s too old for there to be a complete record of exactly where all the tunnels are, it’s going to be very difficult to map or navigate.