Walk through a chilly cave to a blue lake
In these limestone caves, lit paths lead you past stalactites to underground lakes of clear blue water. The air inside stays cool all year, so bring a light jacket even in summer. Caves can close after heavy rain, so check the official site on the day you go.
Three of the cave's underground lakes are open to visitors, and the deepest of them, about 98 meters deep, shows a deep blue known as Dragon Blue. You can also look straight down into the first lake from a walkway above it. The cave stays around 10°C all year and closes when heavy rain raises the water.
A cave in the hills of western Okayama where a path of about 450 meters winds past stalactites and ends at an underground lake. A small bridge over the water is named after Ryugu, the palace under the sea in a Japanese folk tale. It is about 40 minutes by bus from JR Ikura Station, and the cave may close when heavy rain floods the path.
This cave in Oita, eastern Kyushu, was flooded by ancient eruptions of the Aso volcano and is said to be the longest underwater limestone cave in Japan. From the walkways you look into pools of spring water that glow blue and emerald under the lights. It is open every day of the year.