For only 1,000 yen per person (no matter the age), you can be buried alive below a heavy layer of 43 degree Celcius sand. It was quite cold that day, but the sand didn't feel near 43-degrees.
When you pay, they give you a short robe and send you on your way to the locker room where you strip down to nothing at all except the aforementioned robe. I knotted the robe well as the beach area is well traversed.
When you walk out, the women working there have prepared a body sized well for you to lay in. They use shovels and cover you up and they have you stay there for about 15 minutes. It's supposed to be relaxing, but I didn't care for it, but I thought she said 50 minutes. Maybe because it was so much like being buried alive.
When a particular area isn't being used, they fill it with very hot water. I assume it's heated by the same steam vents that head the onsens in the area. Beppu is, after all, the Onsen capital of Japan.
When you are finished, the women tell you to lift your arms up out of the sand and then push yourself up. There is a shower just outside of the locker room where you shower yourself off and toss your robe into a bin. Next, you walk into another locker room with soap and shampoo to clean off before slipping into a bath.This foot bath is adjacent to the sand bath. It is sooo hot! I almost cried when I put my feet in.