Japan has around 27,000 natural hot spring sources, but not every steaming pool qualifies as an onsen. To earn the name, a spring has to meet legal standards for temperature and mineral content. Once you arrive at one, a different set of rules kicks in - and for many visitors, especially Americans, they come as a surprise within minutes of walking through the door.

Start with your shoes. At the entrance and outside the changing rooms there will be a cubby or designated spot, and outdoor footwear stays there. Japan is one of the world's safest countries, so you can leave your sneakers without worry - nobody is walking off with them.

The bigger shock for many first-timers is nudity. At most gender-separated communal onsens, swimsuits are not allowed, nor are towels or clothing in the water. Fabrics can carry detergent residue and debris into the baths, which can force the facility to drain and clean them. You can bring a small towel in with you, but it should never touch the water - locals balance it on their heads or leave it on the bath's edge. Note that some mixed-gender baths permit or even require modesty wraps called yuami-gi, so watch what other bathers are doing.

Before you soak, you wash - properly. A quick rinse doesn't cut it. Sit at the provided shower station, lather up thoroughly with soap, scrub, and rinse completely, removing makeup and tying long hair back. It's not a suggestion; it's the entry requirement.

Tattoos deserve advance research. Traditionally linked to the yakuza, they're still banned at some onsens, while others accept them if covered with waterproof patches or restrict tattooed guests to private baths. Attitudes are softening, but policies vary widely, so check before you go or ask a staff member.

Finally, keep it quiet. An onsen is a shared, calm space, closer in spirit to a train carriage than a resort hot tub. Loud chat, splashing, and phones are all out - photographing other bathers is strictly forbidden.

Onsen come in many forms: indoor and outdoor baths, mixed-gender baths, foot baths, and private reservable baths if communal nudity feels like too much on day one. If you're ever unsure, the safest strategy is to watch the regulars, read the posted signs, and ask an employee. A small amount of preparation means you can actually relax in water that's been drawing visitors for centuries - rather than spend the visit wondering which rule you're breaking.