fwiw, this story is also covered here, imho a more credible source: https://www.aljazeera.com/news/2024/2/1/palestinians-demand-international-inquiry-after-mass-grave-found-in-gaza

gotta admit, that is a lot safer approach than trying some shit on the real thing

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.