I understood why when I saw this page, called Top ten places to go skinny-dipping...
For those coming from this web site and interested in onsen, follow directly this link : www.secret-japan.com/onsen - and keep in mind you will find lots of information on hot springs on the guides available here, too...
Top ten places to go skinny-dipping
Traditional onsen, Japan –
Japan’s tradition of hot-spring bathing is enshrined in the nation’s culture. Located on the Pacific Ring of Fire, Japan bubbles with volcanic activity and there are thousands of bathing houses serving locals and tourists alike. The Japanese view onsen as the perfect stress buster, a place to unwind and shake off the city’s shackles. Naturally, you can only do that if you’re naked. If you’re prudish most onsen offer single-sex bathing.
To help you plan ahead, www.secret-japan.com has a database of over 100 onsen from around Japan, complete with entry fees and opening times.