Naked truth: Volcanically active Japan has thousands of natural hot springs traditionally enjoyed au naturel. One Osaka onsen has taken that concept to the extreme, creating an over-the-top, around-the-naked-world-in-eight-floors theme park. The complex is divided into themes by country—Finland, Greece, Spain, France, Ancient Rome, and so on. You'll soak amid a replica of the Trevi Fountain, in a hammam-meets-Disney stone bath, and over an Atlantis-themed tank full of tropical fish. Naturally, all baths and body soaks are done in the nude, but—bummer alert—men and women are confined to separate floors. On the bright side, you're given pj's to wear when you walk around the common areas—pink for girls, blue for boys.

Bodies of evidence: Despite its name, SpaWorld is favored by locals—and it's popular. Expect to be hanging with lots and lots of naked Japanese people.

Best reason to bring a towel: The ones they supply are meant for locals. Hearty Gaijins may find the in-house towels only cover one cheek.

Don't forget your: Japanese-English dictionary. This will be especially useful for translating the following: "Pardon me. I'm straight, but may I join your all-male naked pool party?"

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Naked truth: Japan dedicates an entire annual festival to the glory of nakedness. Oddly, this event takes place in wintertime. No matter, the annual Hadaka Matsuri, or "naked man" festivals, draw massive crowds throughout the country. Though the specifics vary by event and venue, the general idea is for lots of Japanese men and boys in nothing but loincloths to run to a shrine where they receive a water purification cleansing. Tip: The full monty is frowned upon—a thousand men in glorified thongs is one thing, but this is an age-old rite of passage.

Bodies of evidence: Imagine running with the bulls. Now instead of bulls, there are a thousand Japanese men dressed as sumo wrestlers. And instead of fear of being gored, there's the fear of not tying that T-shape cloth properly.

Best reason to bring a towel: To wipe your feet. Slipping on all that poured water is a real concern, and you don't have a whole lot of cushioning for your fall.

Don't forget your: Gore-Tex. Once all the fun is over, you'll remember this is still Japan in January, and hypothermia—and shrinkage—will be a cruel reality.