This tunnel, complete with tracks, is quite deep under the hotel/bath complex and is where the waters from eight hot springs in the area are piped. I didn't understand a lot of what the engineer was explaining to us, but it was fascinating to see how it all happens. It was also incredibly hot and humid down there, in addition to pretty dark in most places. We climbed up and down rickety staircases, went into caverns and chambers and were so close to the subway the walls were shaking at one point. Somehow I don't think I'd ever get a tour like this in the States!