First of all, the "underground city" is a piece of backdrop that is for some reason put far below the ground!
But since it's a backdrop, getting close means it will unload, thus resulting in not being able to "explore it" - cant take a closer look
But turns out, @cat.exe has a trick up her sleeve - if you get to a specific location close to Release, you can almost touch the underground city!
Our jorney begins at the top of Allcom - You can glide here from the upper rooms of Anansi, but since it takes a long time, I spared this time by teleporting to a timetrial in place using MIK TP.
We glide across a busy street onto the closest building there
We continue gliding towards release, I put a waypoint to guide me
Everthing starts to flicker here, but it doesnt do any damage :)
Then, when you cannot proceed any further, glide town towards a black building close to triangle/trapezoid building, to the left of my waypoint marker. Aim at it's lower right corner, and keep this direction until you cross the street level
Now, look around - do you see it?
You still cant get too close, or it unloads, if it does so, back out until it's back in sight.
Thanks to @cat.exe for this finding!