Japan has thousands of hot spring towns, and almost none of them have a page in English. This is a catalogue of them: where they are, what the water is like, and what actually happens when you stay at one.
1,598 towns catalogued · 652 with an English name · 429 with the water type identified
An onsen is a bath fed by naturally heated spring water, and the rules around it are simple once someone tells you what they are. You wash first, you go in with nothing on, and you keep your towel out of the water.