I agree with Kallian. Run it and tell us how it all went down.
I will make a few comments (and that's all they are) that you can use to help you create a better story:
I never went into detail the extent of mankind's development before he fell, so go ahead and assume what you like. I will say that an underground tunnel across the Atlantic is a mighty venture. It also had to have been one hell of a construct since nothing else of man survived either...no temples, no pyramids, not even Mount Rushmore. Of course..."ROBOTS" kind of explains a lot of that.
It's doubtful the tunnel would ferry cars. They thought of this when they planned the "chunnel." What happens is a massive build-up of carbon monoxide from gas burning vehicles would have no way of escaping without an extremely costly circulation system. Bullet trains would be the most likely option. Plus the fact that even with three cities in this tunnel, no car ever built could drive that distance before running out of fuel or electricity.
Chances are such a tunnel would link from New York to England or France, not Spain. Think of this politically.
Oxygen is created by plants utilizing photosynthesis. So plant life in this tunnel or this far down would not produce oxygen naturally...unless they were magical. Also...regardless of magic, though pure water is transparent, ocean water is not. There is no sunlight at the bottom of the ocean, so no glass walls. And its flora, not fauna...fairly certain fauna don't produce oxygen, they expel carbon dioxide and fart methane...thats about it.
No one would build a tunnel perfectly straight that length. It would have to follow terrain of the Atlantic. You should ride the MTR station in Hong Kong sometime.
If you create an underwater city, name one Star Oceana. Call it a personal favor.
In Grade 10, we were given an assignment to create a futuristic city. Everyone expected me to create some space city, so I threw them a curve and did one underwater, Star Oceana. It was a three domed city at the bottom of the Atlantic with two industrial facilities and a residential one. It also had bubble nets which kept in their food supply. It ran natural sunlight from artificial bulbs that helped psychologically and assisted the growth of plant life inside the domes. It ran a single massive tunnel simply called "the cable" that connected between the city to Sao Miguel in the Azores. I picked the Azores because it was smack in the middle of the Atlantic, allowing a relatively short distance for the tunnel. The cable had a single two-rail train system as well as several larger vacuum tubes for supplies and the ferrying of mineral ores, which was the city's primary export. There was a massive network underneath the city where the titanium and silver ores were dug. There was no glass on the domes. They were triple hulled and reinforced by the lovely invented material "plastisteel".
Did I mention I was 15 when I wrote this?
I drew like 9 images, a cross section of the "cable" as well as a forward plan on how the city would look in 100 years time. I got 60 out of 30 (yup) and nudged my final grade in Social Studies to a B.
Tap that story for all its worth...
â€œI despise stallions. I ride them as I ride an ugly noble women--smiles for appearances and politics only. Stallions. All personality with no depth. They prance like ponies and trot with arrogance. I always say, respect your mules most of all. A stallion may lead with the king and gather the eyes, but a mule will follow behindâ€¦and carry your kingdom.â€