Check out this 75 minute float at Float Boston, a flotation therapy experience in Magoun Square, Somerville. No expiration date given.

From their website: "A float tank is a small shallow pool filled with water nearly saturated with Epsom salt, so you float completely effortlessly. The tank is kept warm, body temperature, and it's enclosed so it's completely dark and quiet. It's completely relaxing."

http://www.floatboston.com/