The tunnels under Odessa are WILD. I realize plenty (most?) of you guys run on metric mesurements, but this should work for everyone -- there's more distance of tunnels under Odessa than the distance between Paris and Kiev. Their layout is weird too -- look at this.
The reason for that is that they weren't mostly to get from point A to point B. Odessa is mostly built of limestone, at least historically. It's also mostly built mostly ON limestone. So a lot of this was quarrying as well as for more standard tunnel purposes. Of course, they were used by the Soviet troops during WWII, as well as some Axis guys, and of course smugglers and so on throughout their long and wind-y history, and today they're a (possibly very dangerous) tourist attraction. For more info, go here.