So had a new experience and went to the Salt Cave in Bethesda (a second choice if you can't get to the secret superhero crime-fighting Bat Cave).
It was a large room filled with Himalayan salt for experiencing all the spa's "healing" effects.
Supposed to be good to breath in all the minerals infused from the pink salt "micro climate" air and also for exposure to the skin.
It was fairly dark in the cave except for some recessed lighting behind some of the salt stone walls.
The chairs were these weird beach chairs that you could recline in, but it would've definitely been worth it to have some fancier cushioned seating to more fully relax in and to match the spa ambiance that they were selling.
In the background was relaxing meditation music and we were closed in the salt cave for 45-minute to enjoy the dark quiet salt effects.
It must've relaxed me, because when I got home, I had a Buddha face on and took a nice post-spa afternoon nap. ;-)
(Source Photo: Andy Blumenthal)