Do I need a guide?
You do not need a guide. Stop asking around for one right now. You don't need even need to speak French. If you possess the ability to read a map and some common sense you'll be fine. In fact, you probably don't even want a guide. Not only is it kind of lame, but a local is not going to want to hoof it around for hours visiting all the places they’ve seen a million times. Locals generally go to the catas to party, or just to relax and hang out with their friends. If they’re doing something else it’s probably making art or working on opening something up, both of which are going to entail staying in one place for hours while you're chomping at the bit to go take off down the next tunnel.
If, for some reason, you are still insisting on asking around for a guide, you should probably be offering to pay. Why do you think someone would want to schlep around in the dark with a stranger for free? Actually, come to think of it, hire me. I'll do it for plane fare, accommodation, and a $500 day rate.
Well, you will probably bump your head a couple times. You might twist an ankle if you aren't careful. I've heard that there is a small but real danger of structural collapse, although the Parisians have spent the last 234 years stabilizing the network.
You'll also get your feet wet. If you don't want to get your feet wet, you're going to need either hip waders or chest waders, depending on your height. I have yet to see someone go in with calf-high rubber boots and come out with dry feet. With careful navigation you can pretty much avoid the thigh-level water, but not the knee-level water. While most of the tunnels aren't flooded, this is a pretty commonplace scene:
You'll probably also get dirty, although that might just be me. Somehow, the French have this amazing ability to spend hours in a dank, dirty tunnel network and still emerge looking like they just stepped out of a Yves St. Laurent catalog. I come out looking like this.
But isn't this dangerous?
Well, no, not really. I'd say it's marginally more dangerous than getting out of bed in the morning and going about your day, but not by a whole lot. The biggest thing is this - a lot can go wrong just going about your day. You could accidentally step in a hole and break your ankle, or run into some mean people and get mugged, or get too drunk and not be able to find your way home. But you're not too worried about this - you call an ambulance, or the police, or a cab. And anyway, how often does this really happen?
The biggest thing in the catas is that there's no "Esc" key. If you break your ankle, you're going to have to figure out how to wind your way back through the tunnels for a couple miles to the entrance in horrific pain before you can call a doctor. It's not that much different from going backcountry camping in this regard, although you certainly can't call for a Medivac or anything.
But isn’t this illegal?
Sure. Being illegal it what makes the place so great. With legality comes security guards, "don't touch" signs, 6 Euro hot dogs, and all that garbage.And anyway, it’s not THAT illegal. Even if you do run into cops (which does happen) you won’t go to jail or get deported or anything. You'll just get a fine. I've heard different amounts for this fine, with the highest being 120 Euros, which is probably what you'd pay just going sightseeing for a couple days anyway. Plus, you know, this is
Will I run into people?
Most likely. There's a lot of folks in the catas, especially on weekends. And people come from all over the world. I've spent a total of about 100 hours down there, and have met (in addition to countless French), Japanese, Swedes, Koreans, Germans, British, Australians, Canadians, Americans, and a hound dog.
You're most likely to run into people around La Plage, which is kind of the Times Square of the catacombs and about a 30-minute walk from the main entrance. These will probably be groups of mostly-young, mostly-male French folks hanging out. People are generally nice enough, but the vibe is pretty much "we're all here to do our own thing." A simple "Bonsouir" is usually sufficient communication, although I've had conversations with people. If you speak French you'll obviously have an easier time, but plenty of people speak English. You might also run into some foreigners, who will probably be taking pictures.
If you get deeper into the catas, you're more likely to run across more hardcore cataphiles. These folks will probably be a bit older (30s or even 40s) and dressed more for a caving expedition than a party. This is what a cataphile generally looks like (on the left):
The older cataphiles will know a lot more about stuff not on the map or usually closed things that happen to be open at the moment. With both groups, offering booze is always a good way to make friends.
Light - this is the most important thing. Bring three light source (same as for caving), with extra batteries and whatnot. If you run out of light, especially in a part of the catas that isn’t frequented very much, you’re somewhat fucked.
The Map - regular cataphiles don't need one, but you sure do. Think a fairly complex Dungeons and Dragons module. That's what you're in. If, for some reason, you lose the map you probably won’t die. Your best bet is to wander around until you find somewhere that looks interesting or has a lot of art and wait there. People will be along sometime.
There's a few different maps, but they're all more or less the same. When I went for the first time in January 2006 I got a .pdf version from some cataphile friends that I printed out at printing shop (It measured about two feet by three feet), and have used that version ever since, mostly just because it's what I'm used to. But you, full denizen of the digital age, don't even have to worry about having a hookup anymore. You can now download a copy (in English no less!) here.
The site's a little hard navigate, but hey: if you can't make your way around a website, you don't really have any business trying to make your way around the catacombs. Whatever map you end up using will probably have an inaccuracy or two, and won't cover the entire network (they generally omit a lot of the southern part), but are incredibly comprehensive overall and will be more than sufficient for getting you around. Some people laminate theirs to keep it from getting wet or dirty (which is almost certainly will), or sometimes cut it up and make a little book out of it instead of keeping the huge version. I just take a spare or two.
A tip: printing large format in France in incredibly expensive - print a few copies in Kinkos or Staples before you go - it should only cost you a few bucks each in Black and White (you don't need color).
Water - if you're planning a trip of any length, this will probably be your limiting factor as water is pretty heavy. But again, if you run out of water you won’t die – there’s a bunch of flooded tunnels where you can get a drink (flooded tunnels are even indicated on the map). Now, I should mention I have no idea how potable this water is and I have heard stories of people getting sick from drinking it. But if you're really stuck, as it’s just groundwater, I’m guessing it’ll most likely be OK. It probably won’t even be that gross as long as someone hasn't walked through it lately and kicked up a bunch of dirt.
Food - think camping provisions if you're going for any length of time. If you're just ducking in for the night, by all means prepare a 7-course continental dinner or whatever else you can haul through the tunnels for a half-mile or so, which is about how long it'll take before you get somewhere you can sit.
Sleeping Gear - I highly recommend sleeping in the catacombs, but you've got to do it right. I've spent two nights down there - one of the best, and one of the most miserable sleeps of my life.
This is how to do it. The second time I brought a hammock. This was awesome. I still slept in my sweater, pants, and sleeping bag but didn't need a hat or anything. There's a bunch of places where people have put hammock hooks. We slept in the Cabinet Minerologic, which has the disadvantage of being somewhat heavily trafficked, but was the only place we knew of with hammock hooks (although I'm pretty sure there's some in the Salle Rouge and Salle Pi also). You can get a travel hammock that's really light and packs down to the size of a softball for about 50 bucks.
I suppose you should probably bring a first aid kit and such also. I can't say I’ve ever brought one myself, but it seems like a good policy.
Generally speaking, it's funnest just to wander around and find awesome stuff (and there's a bunch of awesome stuff). There's lots marked on the map - go find it and see if it's cool. If you really want a destination guide, some of the most popular follow. Consider this a spoiler alert. I should also note that this only refers to the "Great Southern Network," or the largest quarry network that runs mainly underneath most of XIV arrondissement and parts of the neighboring arrondissements. There's also other, smaller networks throughout Paris and the suburbs.
If you're into art - go check out La Plage, Le Cellier, Gallarie des Promos and the Castle Room
If you're into history - go check out the German Bunker, and try to get to the Val de Grace although it's kind of a pain. Also make sure to read up on the inscriptions (it's on the English map), and pay attention to those. If the Free French bunker is open get to that also, but it's usually closed.
If you're into bones and creepy stuff - go check out the Ossuaries and the Tombe Philibert. Also go on the official tour.
If you're into cool spaces and architecture - go check out the Salle Z, the Abri Laval, and definitely try to get to the Aqueduct too, but that's almost always closed. Also the Cube Room I think is kind of cool.
If you have a limited amount of time - you should probably enter and head straight to La Plage, Le Cellier, the Salle Marie Rose, and the Castle room. They’re all reasonably near the main entrance and pretty close together. You can be visit them all and still be in-and-out in a few hours.
(Cata pros - feel free to chime in on any of the above. I'm sure there's a lot of stuff I'm forgetting or haven't been to).
Time - for some reason, time seems to go really quickly in the catas. You'll check your watch thinking you've been in there for a few hours and it's already the next day.
Weather - one of the wonderful things is that the weather is completely consistent year-round. About 55 degrees (Fahrenheit) and very, very humid. This has the practical effect of making you kind of hot when you're walking around, but quickly cooling you down when you stop to rest. As such, layers that you can easily put on and take off are a good idea.
Taking care of business - you can pretty much pee anywhere reasonably out-of-the-way, it won't be too long before it gets filtered into the groundwater. For a #2, you should probably go to a dead-end tunnel and bury it, although as it's not going to decompose anytime soon try to abstain if you can (it's also just plain nasty to take a poo down there). It's also obviously good form to pack out all your garbage. The cataphiles are actually on a bit of an anti-littering kick right now, and have started organizing cleanups.
So how do I get there?
So that's about it. The catas are loads of fun and, at least for me, the coolest place in the world (and I've been a lot of places). I highly recommend checking them out.