Hot water percolates up out of the ground from one end of Japan to the other. The Japanese word for a hot spring is onsen, and there are more than 3000 of them in the country, more than anywhere else on earth – it’s like Iceland on steroids. In Japan, it is not an unusual or even decadent way to pass time soaking in a tub of bubbling hot water. Here are some that are considered to be the holy grail of the onsen world.