The thing about scale is that it can be hard to communicate in words. Ten kilometers, a hundred kilometers, a thousand kilometers, your mind struggles to give them appropriate impact.
I wrote up an Atlas entry today on the Odessa Catacombs, the largest catacomb system in the world. Besides having housed both hundreds of armed Nazi-killing soviet partisans during World War II (who were literally sneaking about right below the fascists and who even managed to occasionally blow up a Nazi building here and there), and having served as trade routes for smugglers and criminals, the tunnels sheer scale is astonishing: twenty five hundred kilometers of tunnel to be exact.
So much tunnel, with so many twisting, cold, dark passages, that people regularly get lost in them (teens who think they know what they are doing generally) and have to be rescued by one of the many semi-pro catacomb exploration groups that exist in Odessa. In 2005 a girl who was partying with her friends in the catacombs, got separated, and died after three days of wandering through the tunnels. It took two years for the police to finally locate, and retrieve the body. Not a good place to get lost.
Back to the size. I have to say even as I wrote all this up, I never really grasped how big 2,500 kilometers of tunnel really is. Even as I noted that 2,500 kilometers is more than the distance from Odessa to Paris, that barely made sense, how can all that distance be contained in a network of tunnels under a single city? It didn't really sink in... not until I saw the maps.
The maps of the Odessa Catacombs look like some fractal Borgesian nightmare, like something an obsessive compulsive Dungeons and Dragons player might draw. Like the fractal branching in the lungs, these tunnels can occupy twenty five hundred kilometers of space, because they nearly defy the idea of size, they just fold back in on themselves, in ever smaller paths. (See Mandlebrot on the art of roughness and fractals for things that really do defy the idea of surface area.)
This of course make the catacombs a very inefficient means of travel and 1995, in honor of the 200th anniversary of Odessa, the record for the longest underground journey in the catacombs was broken by a 27 hour continuous journey of over forty kilometers. Had they walked in a straight line the journey would have been only nine and a half kilometers long. Below are some more of the maps, the middle one made by an amatuer exploring the catacombs. Let the mind boggling begin.