I went to Notre Dame, but I didn’t actually go to Notre Dame. I just walked past it on my way to the ruins beneath it. I thought about going inside the cathedral afterwards, but the crowd was just way too big. I did snap a couple quick pictures, though.
The Archeological Crypts of Paris contain remnants of both Roman-era Lutetia (as it was called then) and Medieval-era Paris. And hardly anyone knows they’re there, which means you essentially have the place to yourself as you walk around. I occasionally passed someone or saw them off in a different corner of the place, but I pretty much got to walk around listening to my audio all on my own.
Before you start looking at the ruins themselves, they have text panels and models that tell you about the history of Paris, starting with when it was just a bunch of Parisian (as in the tribe, not in the modern sense of the word) farmers living in a little village. Then they go through how the city evolved under Roman occupation and eventually during medieval France.
It’s amazing how heavily altered the landscape of Paris is. The topography has been changed by humans so much that places that used to be other islands in the Seine are just part of the banks now (only two islands remain now). Part of the ruins down there are actually an old wharf–160 ft (50 m) away from where the riverbank is now. They have a sequence of projections on it and the screen behind it to really help you get a sense of how it used to look.
They have a 3D recreation of Notre Dame at various stages on touch screens that you can spin around. You should check out the website in the picture. It’s part of an impressive project.
It’s staggering how much they know about the ruins. Reading the panels and listening to the audio guide, you really got the sense that they had to leave a lot out to keep the attention of most visitors. They were even able to peg one of the buildings down as a specific medieval children’s home that started out great but eventually fell into disrepair with 1/3 of the kids living their dying from various communicable diseases.
The last thing you see on the way out is the remnants of the Roman baths. They were shut down some time after the Romans left because the people who lived there didn’t keep up with maintenance on the aqueducts needed to transport all that water there.