Hi Shari2,
As I understand it’s part of the Lotus Spa. Here is a link you can check out;
https://www.princess.com/ships-and-experience/ships/products/joyful-rejuvenation/the-enclave-at-lotus-spa.html
We discovered it on a cruise in the Mediterranean aboard the Star Princess. We were given a ship tour on our first day and they shared that for a very limited number of guests and on a first-come-first-serve basis you could purchase access to an exclusive part of the Spa that featured a number of thermal beds. We found it very relaxing after a day of adventure on shore. It seems to me it was about $150 per person (2011) for a 12 day cruise. There were perhaps 20 - 40 spots available. We went for it and were so glad we did.
This feature wasn’t available on our next cruise but it is available on the cruise we booked for next year on the Sea Princess. Unfortunately you cannot book it in advance and they won’t even tell us what it would cost. We’re just hoping for the best.
I hope this helps.
Take care,
Craig