Michael If I'm not mistaken, the obsession of climbing indoors on artificial walls must have originated in Germany, or how else would you explain that I've been doing that since the 90ies, back then at the Thalkirchen climbing center in Munich? In the U.S., the sport is called "indoor climbing" and there's a micro-chain gym called "Planet Granite" in San Francisco and the surrounding Bay Area that offers several fitness clubs with rock climbing walls. After a hard day's work, it's quite common to meet fellow techies there, and ironically also often German-speaking immigrants. Almost every Wednesday, I've been hanging out there with our friends Bettina, Miguel, Chris and Tranquilla for quite some time.
I remember that back in Munich, we just showed up at the outdoor climbing facility in Thalkirchen with climbing shoes, a rope and a couple of hooks, and off we went up the climbing walls. In the U.S., with its litigious society, it's more strictly regulated. When I showed up at Planet Granite many years ago for the first time, the staff there wanted to talk me into pay for a beginner's climbing class to learn belay techniques, which I already had on auto-pilot because I had been using them successfully for many years. Luckily, I convinced them to start their certification process for free with me, by me showing them how to tighten knots and using their belay device safely.
Belaying isn't hard and an average person can learn the knots and rope techniques within a couple of minutes. But I demonstrated the procedure slightly differently to them, boldly deviating from the climbing gym's holy scripture, and they failed me the first time! The second time, I did it exactly as they wanted and passed. Ever since, my climbing belt has a red card hanging from it, which authorizes me to belay other climbers. It is only applicable to the so-called top rope technique, which uses a fixed rope already installed on the route. With this, the belayer constantly keeps tightening the rope, so that the climber will never fall more than a foot or two, even if they slip badly. But with the red card license, I'm not permitted to bring my own rope and lead, or belay someone leading, for that matter. For this, they require another, more advanced certification, and upon passing, you'll proudly display a green laminated card bouncing from your belt. Clearly ridiculous, but, hey, who am I to complain. Every installed rope comes with a rope-stopping device called a Grigri, which is dead-simple to use and allegedly provides better safety than the half-mast knots we used instead in Munich. Patrolling staff members make sure everyone using the Grigri holds the outcoming rope tighly in their hand and downwards, to make sure the Grigri's safety is enabled, and if they spot someone who doesn't, they'll approach them quickly and remind them in no uncertain terms of the correct procedure.
What's surprising is that the American grading system for climbing routes differs significantly from the "UIAA" system used in Germany. From way back when, I remembered that a "4" is a route with handles so generous that you can just walk up there like you're climbing a ladder. A "5" offers less obvious ways to hold on and climbing a "6" actually requires knowing some climbing technique, or there's no way to proceed. The American grading system is quite different, as documented in Klettergradvergleichstabelle auf Wikipedia, and its grades 5.4 to 5.6 correspond to the UIAA rating of 4, the slightly higher 5.7 to 5.9 to the UIAA 5, and more advanced 5.10a, 5.10b, 5.10c, 5.10d and 5.11a to the UIAA 6.
There's more highly graded routes, but according to my experience, those walls can only be scaled by Spiderman-like creatures. What's also interesting is that climbing as a hobby in the US also seems to serve as a way to meet new people, the climbing gym even has special events for singles to hook up with new prospective mates!