The real world ends beyond a thick, black curtain. On the other side is one of Japan’s last remaining hihōkan (sex museum, literally “treasure palace”) in the faded resort town of Atami, Shizuoka Prefecture — a strange, dimly lit space of questionable morals and dated fantasies.

After handing over my ticket and entering, my eyes adjust to the dark and slowly focus on a bare-breasted mermaid sitting serenely atop a rock, seductively strumming a harp. But, as I soon discover, this is merely foreplay.

Rarely mentioned in English travel guides, Japan’s sex museums are neither easy to access nor actively promoted by most locals. Yet they are interesting attractions that give a rare glimpse into the decadence of 1970’s and ’80s — an era of incredible economic growth, social change and, it would seem, business-minded perverts.

Atami, a picturesque coastal resort town in Shizuoka, famed for its hot springs, became a well-known getaway during this period. Hotels began popping up in the early 1980s, and attractions, such as the Atami Hihokan, soon followed. What better way to even out the wabi-sabi (rustic beauty) of traditional hotel rooms than with a pit-stop at a utopia of kitsch reveries.

The savvy owners have made this sex museum the easiest of all to get to. For ¥1,800 you can get a museum ticket and a round trip on the Atami Ropeway cable car directly to the entrance, perched at the top of a small cliff (“Lovers Point”) with view of Sagami Bay. Passing through a black curtain, visitors to the museum can view erotic 3-D projections or peep voyeuristically through holes at more projections of women in costume. In one room, classic paintings morph into black-light reinterpretations. I watch an older gentlemen step in front of Leonardo da Vinci’s “Mona Lisa” as she morphs into a crass, neon animation with blonde hair and fishnet stockings, tongue poking out playfully. “That’s not funny,” he says over the cackles of other visitors — a lie as transparent as the negligee of a Marilyn Monroe figure nearby.

Heading toward the exit, I pass a large plastic posterior — with an eye on each buttock — which winks cheekily at me before releasing a gust of wind in my face.

To make sense of what I’d just seen, I visit Kyoichi Tsuzuki in his Tokyo studio. He has over 20 years experience with these types of off-the-wall attractions known as chin spots (literally, “strange spot”) and is the author of the chin-spotters bible, “Chin Nihon Kiko” (“Roadside Japan”).

To understand the hihōkan phenomenon, Tsuzuki takes me back to the postwar era, when a Japanese pearl tycoon, Mr. Matsuno, traveled across the U.S., selling his nacreous wares at trade shows. It was on this trip that he witnessed America’s obsession with the car and the kitsch roadside stations that serviced them. Imagine it: a vibrant diner with checkerboard floors, full of pretty ladies in pastel swing skirts, finessing their poodle cuts in chrome reflections. Men outside filled futuristic-looking Buicks, clicking their fingers in time to rock ‘n’ roll hits doo-wopping out of a jukebox.

It was love at first sight — the old pearl tycoon was obsessed. He was so inspired by what he saw that when he arrived back in Japan he set about birthing his own version of 1950s America in Ise, Mie Prefecture. And that is how the Ise service station came to be, complete with its U.S.-style gas pumps, motel and bowling alley.

Matsuno, however, wasn’t just a pearl dealer, he had amassed quite a collection of sexual oddities over the years. He received such overwhelmingly positive feedback after holding an exhibition of these objects at his station that — against the advice of colleagues — he closed the bowling alley and reopened it in 1972 as Ise Gansōkokusaikan, Japan’s first true sex museum. (There was an earlier sexually themed “museum” in Tokushima called Omejin, but Tsuzuki doesn’t really consider it a part of the story.)

Unfortunately, Matsuno was more of a fanatic than a businessman and others quickly cashed in on the popularity of his idea. Although the Ise building is long gone, it lives on in those museums that used Matsuno’s vision as a template, even down to the sickly tailed serif now synonymous with sex-museum signage.

If the gaudiness of Atami Hihokan doesn’t hit the spot, perhaps the Inochi to Sei Museum (Life and Sex Museum) in Ikaho, Gunma Prefecture, might tickle your pickle. Close to the Mount Haruna and the iron-rich waters of Ikaho Onsen, it’s a perfect daytrip from Tokyo.

The museum aims to school visitors about all things sexual, from contraception and childbearing to AIDS. But let’s be clear, despite its educational front, the Museum of Life and Death is definitely a hihōkan, with racy exhibits on its second floor.

Again, interactive elements are plentiful. Visitors can wear pregnancy suits and sit in gynecology chairs or even don death robes and lie inside a coffin — perhaps a good place to contemplate the mysteries of la petite mort.

The museum also has dome with 1,500 blue LED lights, a large collection of S&M paraphernalia and figures sculpted by artist Shunpei Toki arranged in cabinets. Made to order, these sculptures represent an unknown collector’s deepest, darkest fantasies — some are shockingly vulgar. Heat rises, and up a thin spiral staircase are rooms — red and black lacquered sex dungeons — which, according to a sign, are available for parties. Paddles and whips dangle from the wall provocatively and, if you want, you’re free to entangle yourself in rope-bondage chairs or use the invitingly laid out beds. I pass.

However, not everyone finds these run-down palaces fascinating. “Locals don’t like to be near them,” says Tsuzuki, recounting interviews he has done with people living around various hihōkan. “To them they are dirty and embarrassing. Even if it’s there for business, people would rather not be around them.”

Whatever your feelings are, there is no stopping the gradual decline of Japan’s sex museums. What was once a relatively successful industry has fizzled into a small stream of (largely) young people looking for tacky thrills.

I heard rumors that Kinugawa Hihoden, built in 1981 in Tochigi Prefecture — the second of Japan’s two authentic sex museums — was closing this year. As of October their official Twitter account states that although they’re still open, they will no longer be adhering to a fixed schedule and punters must call ahead to check.

Feeling like time was running out, I made a trip there on the quaint Tobu Kinugawa Line, which cuts through dense foliage with onsen hotels sporadically emerging through the greenery.

Despite the surrounded beauty, Kinugawa Hihōden is by far the seediest and shabbiest sex museum I’ve had the pleasure of visiting. The standout exhibit for me was the aptly named “Romanvideo Sexy Box Love Cinema,” which was a room with a projection of erotic films and plastic chairs set out in rows. Most exhibits, however, are sexually explicit dioramas — hunky monks and tatami-room fornication — under dingy lighting.

Kinugawa Hihoden’s traditionally Japanese take on things is not so unusual. Katherine Sender, a “communication researcher on sexuality” from The University of Auckland, writes that apart from the odd Marilyn Monroe (who makes appearances in Atami and Kinugawa) Western portrayals of sexuality are lacking at most Asian hihōkan. In fact, primitive artifacts, such as well-endowed haniwa (ritual terracotta clay figures) of the Kofun Period (250-552) and depictions of Imperial and monastery scandals known as shunga feature heavily, alongside shrines related to sex, suspicious looking stones and phallic woodcarvings.

I ask one visitor — a Japanese male in his late 20s — why he enjoyed visiting hihōkan. “It’s like a portal into this very personal, socially forbidden world of taboo that we all suppress,” he says. “It’s an adventure and I’ve been given permission to indulge in it, in the company of others. It’s absurd, weird. It’s fun.”

Tsuzuki agrees. “Hihōkan were a gateway for me into this world of strange, unknown places, and they truly are treasures, as the kanji suggests.”

I couldn’t agree more, but they’re not going to be around for much longer. So get out into the countryside to see the birds and bees — before it’s too late.

Getting there: Atami Hihōkan (atami-hihoukan.jp) is a 2-km walk or taxi ride from Kinomiya Station on the Ito Line. The Life and Sex Museum (inotitosei.jp) is best accessed by car or a 4-km walk from Yagihara Station on the Joetsu Line. Kinugawa Hihōdan (0288-77-0564) is a short walk from Ryuokyo Station on the Aizu Kinugawa Line.

In line with COVID-19 guidelines, the government is strongly requesting that residents and visitors exercise caution if they choose to visit bars, restaurants, music venues and other public spaces.