On my walk along the Cilgerran 'Gorge Trail' you go past the Fforest glamping site, where you will find these 'Onsen Domes' ( https://www.coldatnight.co.uk/onsen-dome)
An 'onsen' is a Japanese hot spring; and the bathing facilities and traditional inns frequently situated around a hot spring.
In Japan the presence of an onsen is often indicated on signs and maps by the symbol above Traditionally, onsens were located outdoors, with particular care taken over the siting and landscaping to create a place for relaxation and quiet reflection in the natural world. There is a strict ritual to the use of an onsen and the conduct of the user, only entering the onsen after showering and being completely clean.