Proposal talk:Tag:man made=grotto
Unnecessary distinction
Contrasting "human-made" to "in mountains that occur naturally should be tagged as" natural=* is not true for most natural=* features. Instead, there is anthropogenic=yes and refitted=yes (from naturaL=sinkhole). The degree of "human altered caves" can be difficult to determine as well.
For your other definition or description, "Adit" is not simply "Holes" (or "in mountains") either. It has to be a horizontal access to a larger tunnel or underground space, as opposed to shafts being vertical. (And for your "for other reasons (storage, extraction of minerals, shelter) ", man_made=adit and man_made=mineshaft are currently defined for mines only)
There is already location=underground for your underground=yes. Although, it is true location=* can be conflicting (with location=*door or location=bridge; and maybe is it possible to show something is on the surface, overground, or overhead in an underground or elevated space?)
You could use eg natural=cave + cave=grotto (similar to sinkhole=*), together with the anthrogenic=yes or refitted=yes. In fact, there are already 17 cave=grotto instances, Most are natural=cave_entrance (do you have to create a man_made=grotto_entrance too???) around 3127232816
3127232816, and 1 natural=cave 437668184
437668184 with natural=cave_entrance 4489347940 4489347940.
