I'm always fascinated with what's underneath modern cities. It's incredible when you look into it how much of the infrastructure is under our fee The third dimension hides sewers, power and water mains, transport systems, old conveyers, bomb shelters and of course historical materials.
My Mum, on her recent visit, discovered and dragged us to the Guildhall in the City of London with its intact section of a Roman amphitheatre. It's amazing what lies beneath!
Anyway, what brought this all to mind is this excellent description of the stuff going on at Kings Cross as they redevelop it for the Channel Tunnel rail link. Really interesting stuff, and very well presented.