Book About the UK and Underground Tunnels

I read this book in middle school and the only thing I can remember about it is that it was based somewhere in the UK (maybe New York). The main character was a man, I think and somehow he ended up in the underground tunnels. After that, I can't remember anything else about the storybut I know I was creeped out.

If it helps, I remember the cover of the book. On the top half of the book, it was the city and a cobblestone path. The bottom was the entrance of the tunnel (it had a green hue to it) with the shadow of the man entering the tunnel.

I would really like to read this book again. It has popped up in my mind over the years and I'd just really like to discover what creeped me out all those years ago.