Ibusuki's fame comes not from its waters, but from its black, volcanically heated sand. This is one of the only times where you don't bathe before entering -- simply don a yukata and plastic slippers, and walk from the changing room outside to the sunaburo (sand bath). There you'll find workers with shovels (ranging from age 16 to 86) who dig a trench for you to lie down in. After you're comfortable, they cover you up and the sunaburo begins.
A sand bath at Ibusuki is for many people one of the highlights of an onsen tour. It's so unique, and the bath is right on the shores of Kagoshima Bay. You can't get further south without being in Okinawa, and those who come this far rarely regret having added Ibusuki to their itinerary.