The one with London.
To know London means not only visit famous places, but also to travel though it tremendous sewer system, which is known to be the most long among all others, and has sophisticated, hence not completely discovered maze of tunnels, which could have been a shelter for those who were on run, but turnout to become a tomb for them.
Destination of such system, besides the obvious one, is to protect noble citizens from possible terror and to hide numerous secret artifacts which are supposed to be hidden from public sight for years.
Building five kilometer section of those sewers, during the first years of construction of the Big Ben, they'd managed to discover that city underneath has kept number of tunnels beyond any reasonable explanation.
Nowadays using sewers would be a great time saver, if intelligence opened public access to it. Because network of tunnels is ten times complex than an underground, and sometimes it's so wide that you can ride a car in it.
Notorious Wiki Leaks database has few maps of northern part of the sewer system, and many believe that strange marks on those maps were the real reason why WikiLeaks founder was arrested in a short time after publication.
I guess that living in a country, you should have tried to reveal its mysteries, at least to know that you live in a country full of legends and ancient history.