There’s an old saw for domestic air travelers in the U.S that if you’re going to hell, you’ll have to change planes in Atlanta. The corollary in Paris? If you’re going to hell, you’re likely to have to change trains at Châtelet. Châtelet and Châtelet Les Halles are theoretically two different stations but they connect through a maze of grubby, poorly lit tunnels, some long enough that moving sidewalks make sense. All in all, three RER lines (A, B, and D) and five métro lines (1, 4, 7, 11, and 14) go through there. Some of the connections are a piece of cake (just walk across the platform to switch from RER A to B); others will leave you swearing that you’ll do any to reroute around it in the future. One cause for optimism? The RATP has a terrific boutique in the station with demitasse sets, t-shirts, umbrellas, and other unforgettable Paris souvenirs. But danged if I can tell you how to find it in that subterranean netherworld.