About

What this is

There is no shortage of onsen listings. What is missing, in English especially, is one that models Japanese hot springs the way Japan does — as water first, and buildings second.

So this site is built on three things most guides flatten into one. A 源泉 is a spring source: a specific hole in the ground, with a temperature, a pH and a chemical analysis. A 施設 is a facility: an inn or a bathhouse, which may draw on several sources, or share one with its neighbours. A 温泉地 is an onsen area: the village or valley that grew up around the water. Kept apart, they answer the questions worth asking — which inn has its own source, which share one, and what each does to the water on the way to the bath.

What we promise

Cited facts are marked, and uncited ones are visibly not. A source marker beside a price means someone read that price off a sign or a website and recorded where. No marker means nobody has, and you should treat it accordingly. Marking everything equally would make a guess look like a receipt.

Every entity shows when a human last checked it. Onsen close, prices rise, and a bath that was 混浴 — mixed bathing — when it was last written up is single-sex by the time you arrive. A date you can see is worth more than a confident page that has quietly gone stale.

Closed facilities keep their pages. Nothing here 404s because a place shut down — you get the page, with a banner saying so. Being told is the point.

AI-checked pages

Some pages carry an “AI-checked · pending human review” badge. It means a script re-read that entry against live sources — the operator's own website, Wikidata, OpenStreetMap, the municipality's pages — cross-checked what they said against each other, and published it only when they agreed. Every field it wrote carries a source you can follow, exactly as on any other page.

It also means no person has stood in that building. That is why the badge is not the same marker as “last verified”: the accent dot belongs to human checking alone, and an AI-checked page never gets one until someone confirms it. When that happens the badge is replaced by a date.

The check is deliberately narrow. It may set a name, a reading, a location, a municipality, a phone number and a website, and whether a place has closed. It is never allowed to touch what the water is, what it costs, when it is open, whether tattoos are permitted, or whether a bath is 混浴 — those are the facts you would be stranded or turned away by, and they wait for a person.

If something on a badged page is wrong, it is our error and not the machine's. The pipeline exists so the site can grow faster than one person can visit; it does not exist to lower what a fact on this site is supposed to mean.

What this is not

It is not a booking site, and it takes no commission from anyone. Nothing here is ranked, promoted, or sponsored.

It is not medical advice. The 適応症 and 禁忌症 — the indications and contraindications a facility is required to post — are reproduced because Japanese law requires that posting, and because leaving out the contraindications would be unsafe. They are not clinical guidance.

It is not finished. The corpus is deliberately small and deep rather than large and thin; a thousand entries with nothing in them would be easy, and worth nothing.