itting atop a rocky outcrop on Japan’s Izu Peninsula, the pool at Kuroneiwa-buro is so close to the Pacific that the waves are only just out of reach as they crash onto the boulders nearby.

The hot spring is in the tiny fishing harbor community of Hokkawa Onsen. It’s the kind of town you read about in stories on the country’s rapidly aging population — a place young people leave in search of better jobs elsewhere.

It’s certainly not a place you’d expect to find a Western tourist on a Monday afternoon. So naturally, I was a bit surprised to see a foreign woman walking into the hot spring facility’s bathing area.

Taking a quick look around, she headed straight for the source, where water gushes out of a pipe at temperatures well over 50 degrees Celsius, or over 122 Fahrenheit.

Without hesitating, she started scooping it up with her hands and throwing it onto her body.

“Hot! Hot! HOT!” she squealed while doing a frantic little hop as her skin quickly turned red.

She was engaging in what’s called kakeyu — the pre-bath rinse. Everyone does it. But somewhere near the hot spring entrance, a well-meaning sign in awkwardly written English seemingly confused her: “Please pour hot water on yourself before bathing.”

Technically correct, she followed the sign’s instructions. Just not the intent.

“Not that water — you’ll burn yourself!” I called out from the hot spring pool. “Use the bucket and scoop water from the bath, not the source.”

She paused and blinked.

“Ohhh. THAT’S what the sign means.”

I witness moments like this all the time while traveling around Japan — small misunderstandings that can ruin a foreign visitor’s entire experience.

And Japan’s famed hot springs, or onsen, are full of invisible tripwires, from nudity to tattoos, along with a host of unspoken protocols in between.

That’s part of what drew me to them. Born and raised in Japan but having spent most of my adult life in the US, I found myself rediscovering onsen culture, almost like a visitor, when I returned.

At first, even bathing naked around strangers felt uncomfortable again. Relearning both the customs and the experience itself made me more curious about this part of my Japanese heritage.

As I learned more, I wanted foreign visitors to enjoy onsen the way they’re meant to be — a relaxing experience, not a survival test.

That is why I became a certified onsen sommelier. Yes, it’s a real thing.

I’ve traveled more than 1,900 miles, from Hokkaido to Okinawa, on a quest to discover the most extraordinary onsen. I’ve even worked in a hot spring inn — that’s how passionate I am about them.

Onsen 101: Why so strict?

While the word “onsen” is sometimes loosely translated as public bath, there are certain boxes one must tick. When we say onsen, we’re referring to the hot spring water that bubbles up or is pumped from deep within the Earth.

As the world’s hot spring capital, Japan has over 25,000 natural thermal water sources. The Japanese take hot springs so seriously that there’s even an Onsen Law. Legally, the water must naturally emerge from the ground at a minimum temperature of 25 C (77 F). Or, if it’s cooler, it needs to contain one of 19 minerals or components — such as sulfur, iron or carbon dioxide — above regulated minimum levels.

Onsen water is also classified by pH, from strongly acidic to strongly alkaline, and most hot spring facilities display a chart highlighting the water’s components and therapeutic properties.

These waters can be enjoyed in single locations in any given town or city, and many onsen inns welcome day visitors for a fee, often under $10. Some destinations even have free public onsen facilities.

Japan is also filled with entire hot spring towns — it’s easy to know you’re in one as the town name usually includes the word “Onsen.” They offer a range of accommodations, from traditional ryokans (inns) to Western-style hotels, all designed around enjoying the healing waters that flow through the area.

Take the town of Fukuji Onsen, about a five-hour train and bus journey from the metropolis of Nagoya. Tucked deep in the mountains, it has just 11 inns, many over 100 years old and set in renovated traditional houses.

At night, the inns glow under a soft, orange light, creating a dreamlike, almost time-traveling atmosphere.

I fell so in love with this town that I decided to get my first onsen job there, and gained an insider’s view of the dos and don’ts that can make or break trips for foreign visitors. I worked at a 170-year-old farmhouse-turned-retreat called Soene, which has just 15 cozy guest rooms and three bubbling onsen areas for guests to bathe in. There’s a staff of around 15 people, and they want nothing more than for visitors to have a supreme experience.

But it can be challenging for them to deal with foreigners, who are unfamiliar with the traditions and customs.

Soene has become a popular spot for international visitors, despite its remoteness. When I worked there seven years ago, the visitor ratio was about seven Japanese to three foreigners. When I went back last year, that ratio seemed to have flipped.

This is in line with what’s happening in the town of Fukuji Onsen itself, which is gaining foreign visitors far faster than it can add English-speaking staff. While the inns are genuinely happy to welcome guests from abroad, they often struggle to comfortably accommodate them.

The main issue? Staying at a traditional onsen inn is a whole other experience compared to a Western hotel. You can easily book a room with Agoda or Booking.com, but what those sites don’t often mention are the inn’s unspoken rules and customs.

First off, onsen inns run on a schedule that feels almost military-like. You check in, have dinner, bathe, eat breakfast and check out at set times. And yes — you must be there right on the dot, sharp!

You may be thinking, “Hey, this is my vacation! Why so strict?”

It all goes back to the Japanese concept of omotenashi, or hospitality.

The inn wants to know exactly when you’ll arrive so the staff can be fully prepared to welcome you and your tea and sweets can be served fresh. Dinner is also served at a set time, so your multi-course meal can be presented in perfect conditions, at the perfect moment.

The baths, too, must be kept immaculate and filled with just the right amount of water, flowing at the ideal temperature, when you arrive.

If you arrive late, that entire, perfect plan is thrown off, and what looks like rigid discipline is really the staff scrambling to preserve their idea of flawless hospitality.

I saw this firsthand. One late morning at Soene, a male colleague came barreling toward me, his face bright red, eyes wide, clearly in full panic mode.

“You go to the woman’s bath right now!” he shouted at the top of his lungs, his words tumbling out so fast it was almost like he was spitting them.

Apparently, he went into the women’s bath to clean it, only to discover the foreign guests were still bathing. Yes, he had unexpectedly come face-to-face with women — in the nude.

I immediately ran in to check and confirmed that we had informed the guests to finish by 10 a.m. for cleaning, and politely apologized for the incident, while also ensuring they left promptly.

At a ryokan, timing is everything. With a small staff and a tight schedule, every minute counts. When schedules aren’t strictly followed, accidents like this can — and do — happen.

Nudity and tattoos: What’s really going on these days?

That takes us to a question many foreigners ask: Why do Japanese people bathe completely nude anyway? Wouldn’t it be more appropriate to wear swimsuits, especially in mixed-gender baths?

In Japan, naked bathing is a centuries-old tradition rooted in health, culture and social norms. Communal nudity was once normal, and mixed-gender baths were common. Plus, inns often didn’t have the resources to provide separate baths.

However, in the 1870s, Western notions of morality led to a nationwide ban on mixed bathing. The tradition quietly survived at some riverside baths, mainly because rural areas were beyond strict enforcement.

But today, most onsens separate men and women. In mixed areas, visitors may use yuami (special garments) or towels for privacy, though there are a few risqué exceptions.

Though rare, the essence of traditional mixed naked bathing — called Konyoku — still endures, especially at onsen with a toji tradition, which means guests spend days, weeks or even months at a destination with the sole purpose of soaking in its mineral-rich waters for health and healing.

Among these is Amagase Onsen, in Oita on southwestern Kyushu island, where a rare riverside bathing tradition dating back to the 1600s still survives.

Open-air baths here sit at the edge of the Kusugawa River, right beside the main road, which is lined with inns, hotels, shops and the town’s iconic bright red bridge. And in the heart of it all sits Jinden-yu, a stone-lined open-air bath with no walls, curtains or partitions. There’s nothing — no trees, grass or rocks — to block the view.

I had heard of this place but seeing it in person was another matter. A dozen people came and went throughout the day, bathing and enjoying the open-air onsen while fully visible to anyone who happened to be walking along the riverwalk, driving by, crossing the bridges, or staying in adjacent hotels.

At the visitor center, staff quietly noted they cannot actively promote the bath, given it sits on that delicate boundary between traditional culture and what might legally be considered indecent exposure.

It helps, though, that Amagase Onsen’s history stretches back over 1,300 years. According to old local tales, “if you dig into the gravel along the riverbank, hot water will spring forth anywhere.”

So people did just that — they dug hollows and stacked stones to form pools. There was little distinction between men and women — bathing was a communal way to share nature’s blessing, with no partitions.

Another famous mixed-gender hot spring is Sukayu Onsen, tucked deep in the Hakkoda Mountains of Aomori, at the northern tip of Honshu, Japan’s main island. It’s known for its massive indoor communal bath and thick, almost otherworldly steam.

A sign outside the bathing area displays the “Three Golden Rules of Mixed Bathing at Sukayu Onsen” — a simple yet powerful guide that reflects the spirit of Japan’s communal bathing tradition:

1. Gentlemen: Eyes off the ladies — No curious staring!

2. Ladies: Eyes off the gentlemen — No curious staring!

3. Everyone: Embrace the bath with an open heart, respect each other, and soak in harmony, no matter your age or gender.

That last one might just be the best advice for any onsen visitor.

Another issue that gets a lot of attention from foreigners is tattoos, as many onsens have signs stating they do not accept guests with ink. This isn’t arbitrary.

Tattoos in Japan have a long, shadowy history. They were once a hallmark of Yakuza (Japanese mafia) membership, their wearers accused of having ties to organized crime. Because of this, many bathing facilities used to turn away guests with tattoos to avoid any trouble.

Today, some reports suggest that only about half of Japan’s Yakuza members are believed to still sport tattoos, as many in the younger generations see them as inconvenient. So simply banning tattoos is no longer a guaranteed way to keep them out.

Meanwhile, tattoos are incredibly common worldwide, making the old rules tricky for modern inns to enforce.

Some onsen have become more tattoo-friendly in line with the changing times, but many continue to ban them simply because it’s easier. There are still Japanese people, especially older generations, who strongly associate tattoos with the Yakuza, and even harmless decorative tattoos on foreigners can feel intimidating for them.

If you have tattoos, there are ways to navigate this.

The website “Tattoo Friendly Onsen” has a large database of onsens that allow tattooed guests. While the numbers are growing, the listings mostly cover larger, well-known onsen towns.

If you stumble upon a small onsen off the tourist path, being upfront could be the best policy. Tell the inn owner about your tattoos before making reservations you can’t cancel. Describe the size and what body parts they’re on. Staff may suggest using waterproof bandages, medical tape for larger designs or suggest booking a private onsen room.

Many inns do have first-come, first-served private baths — perfect for a worry-free soak.

When in doubt, read the air

OK, so you’ve survived the tricky terrain of nudity and tattoos. Next up: the unspoken rules.

In Japan, most people won’t tell you if you’re doing something wrong. This isn’t rudeness — it is, oddly, connected to their sense of hospitality.

Rather than confront you, they’ll quietly move away if your towel dips into the bath, or your long hair is hanging down and dancing with the flow of the water. (Both of these are no-nos.)

Once, I came across a guest walking around the bathing area wearing a pair of the inn’s wooden clogs. I had to let her know they were only for the toilet area. Looking a little embarrassed, she quickly turned on her heels and headed back to the restroom. (In Japan, footwear is assigned to a specific location and should never stray from its designated spot — a quietly enforced law of Japanese life.)

Meanwhile, washing and rinsing your body at the shower station before entering the bath is a must.

“But I just showered in my room!” you may be thinking.

Well, your fellow guests don’t know that so it’s just a nice action to assure everyone the bath water will remain clean.

After your shower, return bathing accessories like buckets, stools and shampoo to their proper places. It’s a small gesture, but it keeps the next person comfortable and reflects the Japanese commitment to hospitality.

Another tricky issue is conduct. Sometimes I see foreign tourists arrive with a full-on “Woohoo!” attitude — seemingly on the verge of cannonballing into the bath, sending splashes flying everywhere as they hit the water.

I get it. I share their excitement and completely understand how they feel. As a matter of fact, many onsen-loving locals do share that feeling. But others are visiting the onsen for relaxation and want to experience quiet, meditative-like moments.

That said, some onsen atmospheres can be fun and lively.

Japanese people often say, “read the air,” meaning notice the mood around you. If everyone is soaking in silence, do the same. If the bath is filled with chatter and laughter, feel free to join in. It’s less about rules and more about harmony, respect, and simply enjoying the shared experience.

Ready to take a dip?

I’ve spent decades crisscrossing Japan, soaking everywhere from misty mountain hideaways to grand historic baths, seeking out some of Japan’s most unusual and distinctive onsen and have visited a few hundred of them.

For those looking for something truly unique, here are my three top experiences.

Shin Tamagawa Onsen

If you think hot springs are for relaxing, Shin Tamagawa Onsen will blow your mind.

Located in Akita prefecture, bathing here feels like rigorous Bushido discipline. The spring, packed with hydrochloric acid, aluminum and iron, has a pH of just 1.2 — about the same as stomach acid. In fact, it’s so acidic that it can corrode a knife overnight.

The facility has retro charm — its numerous wooden baths filled with natural hot spring water. Survival secret: start with diluted water, then move up to 100% pure, and never soak for more than three minutes at a time.

Easy, right? Not when the water stings like fire. On my first attempt, I had to exit the bath after 30 seconds, hissing “ow, ow, ow!”

Meanwhile, an elderly lady, a seasoned pro, calmly soaked in the water nearby, giggling as she watched me jump in and out.

But this water works wonders: it cleanses, improves skin ailments and eases nerve and joint pain.

Hanayama Onsen

Picture a hot spring where the surface hardens overnight into a crust, almost like caramel on a crème brûlée.

That’s what awaits visitors to Hanayama Onsen in Wakayama prefecture every morning.

The water is rich in iron, giving it a deep and muddy reddish color. Its high mineral content— especially calcium and sulfates — causes this thin layer to harden overnight as the water evaporates.

Only those who arrive at the onsen first thing in the morning get the chance to break through this delicate layer and enter. With this in mind, I woke up at 5 a.m. and waited for the bath to open. Then, I had to race against another woman who was already waiting.

We agreed to enjoy it together, cracking the crust with our fingertips, then gently breaking it with our palms, before finally stepping in. As we went through the same careful ritual — very Japanese, in a way — we couldn’t stop giggling, the thin crust cracking into little clumps that twirled and melted into the brown water.

This spring, rich in iron, calcium and magnesium, is known for promoting blood circulation, relieving fatigue, improving cold sensitivity and aiding chronic skin conditions.

Shichirida Onsen

How about a soak in sparkling water? At Shichirida Onsen, in Oita prefecture, naturally carbonated water flows into the baths.

The concentration of carbon dioxide is so high that, without proper ventilation, you could faint. Thankfully, the facility has large exhaust fans, and all the windows are kept open.

The tiny bubbles pop right next to your ears, tickling your cheeks, creating a playful, fizzing symphony. The moment you step in, bubbles cling to every pore as your entire body is literally wrapped in countless tiny beads of carbonated water.

Even though the water is on the cooler side, I could feel my body gradually warming from within. This carbonated spring is said to promote blood circulation and metabolism, relieve fatigue, and help with cold sensitivity —truly a full-body refresh from the inside out. (CNN)

•PHOTO:People bathe in an outdoor onsen on the shore of Lake Akan, in Hokiaido's Akan National Park.

Update news