
Quick answer
Yama no Yado Shinmeikan is Kurokawa's origin-story inn: its cave bath is hand-dug (confirmed in the inn's own Spa Association listing), and the inn stands at the center of the revival that made modern Kurokawa — the story the History & Revival page tells in full. A member inn on the nyūtō tegata circuit.
A fuller guide to this inn — its baths, rooms and how to book — is in preparation and will follow.