The nooses?
Or maybe it is the blood on the wall and floor.

My butcher friend used to hang his kills till they drained out. Wonder why they wouldn't do the same.

Looks like they do.

Not all monsters live in dark places.

Yes, but perhaps also the tile floor and walls? Note the black and white colors, pure Free Masonry. My heart still sinks every time I think of the horrors conducted in these barbaric arenas.

Agreed.

I toured this place in 2015. The guide mentioned the family had hosted a private event there not long before, so I think they could still be doing this sort of thing. She also said many people feel the place is haunted and don't like being there.

The dark staining at the bottom is weird for a pool. You have to keep chlorine in the water so it doesn't grow bacteria. Since chlorine bleaches things, pool water doesn't leave a dark residue. If there was iron in the water there could be rust staining, but it should be evident anywhere the water was in contact for a long time, not just at the very bottom. And of course, they have a cleaning crew that keeps the whole place immaculate, so whatever it is should have been cleaned away to the extent possible without destroying the tile.

Strange to have an underground pool on a huge estate where you could put it outdoors or in a greenhouse. Or all that non-buoyant rope. Who needs long ropes at the BOTTOM of a pool?