Swimming pools are normally constructed empty. They were withstanding surrounding soil before they were filled, and concrete strength increases with age (for about 90 days, typically). On the other hand, a sunken structure like a pool that is roofed over, becomes a “confined space”. Unlike a typical structure, heavier-than-air gases cannot escape from the pool. Such gases could originate from the drain system or flow from leakage outside the pool area. For examples, leaking propane or various gases from sewer lines in the vicinity. A sunken greenhouse would almost certainly be a building code violation for that reason. If you build it, ventilate it by means both active and passive and do not enter if you can’t verify that ventilation is working.

This idea of themed instances is quite common in the fediverse, I’ve noticed. Built in to the user experience is always a “local” feed - which although a user does not have to view it, it is likely to be of interest if one chooses their instance wisely. Likewise, a user’s postings appear to the other users who view the local feed. So if the user is not “in tune” with the leanings of the other users, and posts often, local users will tend to block the out-of-tune user – not because they dislike that user’s content or disapprove, just because they’re not interested and it clogs up their local feed. At least, that has been my experience. And in that context, it is quite difficult to find one’s “home” - there are many servers, but they only advertise their themes in limited ways and there is no centralized shopping market for servers. I suppose it will arise eventually.