Onsen Hoyoland is one of Beppu's more memorable hot-spring experiences, known for its muddy mineral baths and strong sulphur atmosphere. It is rustic rather than refined, which is exactly why some travellers remember it more clearly than the tidier baths.
Read current bathing rules before going, especially around mixed or outdoor areas, and do not expect spa luxury. The appeal is the raw geothermal character: grey mud, steam, mineral smell, and the sense that Beppu's hot-spring culture has a rough edge as well as a polished one.