More water.
My dream house when I was a kid had one. I didn’t have the whole floor plan laid out, but this was a feature I wanted.
There would be a standard above-ground pool, except there would be a waterfall, built into a rock structure. Pumps would take the pool water to the top and send it back down. However, the rock structure would extend back into a hill or something, and if you swam to the bottom of the pool, there would be a sliding door. You would swim down, slide the door back, flooding the chamber inside, and pull the door shut. The water would then drain (and maybe be fed back into the pool somehow?) and then you’d crawl into a hidden cave under the rocks/hill that would have a secret area, possibly including a pool. Though, that would be more of an “underground” pool rather than an “underwater” one. There would also be another entrance to the bunker… which would totally not be a fallout shelter, just a hidden lounge type area. There would be a “cargo entrance” to back a truck up into to bring furniture in, as well. But the idea is, after moving furniture/other big stuff in, you could close a door and seal it back up with dirt to hide that entrance.
Brine pools, it’s much saltier water which doesn’t mix with the water around it.
If it’s isolated from a current it would be filled with denser saltwater with a higher salt concentration until it eventually fills in with sediment
How about a pool of mercury or tetrabromoethane? Who said it has to be non-toxic. Besides, mercury comes with extra bling.
Heavy water
With goo
