Book about boy falling in to catacombs and finding city

[Tiiwee]

So the book im searching is about this boy who falls inside of catacombs while on tour there. Trying to find way out he stumbles to this city underground. I remember finding it from young adult section of library over ten years ago so its quite an old book.

[Kris]

Tiiuza, is the boy with friends (e.g., Stalked in Catacombs-Da#3 by Peter Reese Doyle)?

What happens in the underground city? Who does he meet?

What's the genre or tone - mystery, fantasy/paranormal, horror, etc.?

[Tiiwee]

I think it's fantasy genre. It's not the book you mention.

The people who live in the city are descendants of some people thrown into the catacombs still alive back the time when there was plague.

[Kate]

Could it be Gregor the Overlander?

[Tiiwee]

No. In the start of the book he is in tour in catacombs and he falls in. I guess where he falls is some sort of hole inside the catacombs where they just threw bodies

[Kris]

Is this a zombie story?

Is it set in a famous city like Paris or London?

[Tiiwee]

Nope not a zombie story. It might be set in Paris but not 100% sure.

Right ja have been trying to think what else I remember so here is spoiler alert!

This boy I think has friend with him in there too and somehow he ends up in the pit too. Not sure if he jumps in there or if he falls while trying to help.

Before the city there are these tunnels they go to while trying to find way out.

In the city they believe that hell is up and heaven is underground.

The people in the city have the plague but they look normal as I think somehow they can carry it but not show or suffer from it.

I think the boy/s are stopped from leaving the city but at some point they escape and get back home.

Then I remember there being something about them bringing the plague up and the boy infects his mother with it.

I think there are 2 or 3 books in the serie and it might be written by two authors instead of just one.

I'm trying to think if I can remember more about it.

[Tiiwee]

Oh and I read it somewhere in early 2000 and I think it was already fairly old book then already

[Lobstergirl, au gratin]

Tiiuza, are you still looking for this or did you find it?

[Lobstergirl, au gratin]

No response, moving to Abandoned.