The statistic of low Firefox use is based on accessing US government websites. Could it be that there is significantly LESS government site access by the population of users that prefer Firefox? As a corollary I recently read that game companies observed significantly HIGHER bug reporting from Linux users on Steam, not because there were more Linux-related bugs, but simply because that set of users were more likely to initiate bug reports. Of course Firefox is not Linux and Steam is not the world, but a statistic from a relatively narrow segment of the internet should not be assumed representative of the whole.

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.