I once saw a setup with a whole rack laid down on its back in a tub of mineral oil, like you describe. You could slide individual servers up and set them on th

I once saw a setup with a whole rack laid down on its back in a tub of mineral oil, like you describe. You could slide individual servers up and set them on the top of the enclosure, letting them drip down into the tub. Kind of messy, but not unreasonably so. (This was setup at SC'09 in Portland. I'm impressed that someone went to the trouble to set this thing up just for a few days of a conference.) The oil was actively pumped through a radiator with a fan. Part of the whole point of the system is that, since oil is a better coolant than air, they can run the whole system at a higher ambient temperature, like 80 or 90 degrees F instead of 70-ish. That could reduce energy costs quite a bit. I'm not sure if the oil goes bad eventually and has to be replaced; I could see that being expensive if there isn't a simple way to clean/recycle/reprocess it back into usable oil.