If not for its debaucherous reputation, you might not blink an eye passing the Fuji Building.
From the outside, it looks no different to the skyscrapers that surround it, reports news.com.au.
Looking up, its dishevelled 22-floor facade camouflages itself with its neighbours in Hong Kong's Causeway Bay, despite the suburb's reputation for its luxury malls and boutiques.
But inside tells a different story: Eighteen floors of "one woman brothels".
The Fuji building has been gaining international notoriety for its "high-rise house of prostitution", according to writer Viola Gaskell, who toured the building for the Daily Beast.
"Inside, a line of young to middle-aged men queue for the elevator," she writes.
"My translator and I are the only women around. The bewilderment on the men's faces increases as we ride to the 22nd floor with them."
But it's not difficult to find out more about this sordid spot. YouTube will show you (almost) everything you need to know, despite signs on the buildings walls which warn that filming and uploading video to YouTube is "unethical". Even TripAdvisor has a gallery of images.
Inside, past its tiled entrance, is a stark contrast to the building's bleak facade.
Hallways are decorated with fairy lights outside each room. If the lights are lit, it usually indicates the "woman inside is available but it's not a sure thing". The lights glow in various shades of pink. Signs on the doors read "welcome", or "wait" in various languages.
Another reads: "I'm worth the wait". Strange anime cartoons accompany the messages.
Behind each door lies a different woman. The floors vary in decorative appeal and become less busy the further down you travel.
"On the 22nd floor a single room is 'available'," writes Gaskell.
"I ring the silent doorbell; the door opens revealing a young, pale-skinned, Chinese woman in a white negligee. Her cloudy contacts match the dreamlike, opaque lighting in the room behind her. The room looks clean and well-mirrored."
There are 141 women in all - between five and eight on each floor. Some are from Laos, others are Chinese or Russian. Most are in their twenties, and many of them also live there.
An average visit will cost around 500 Hong Kong dollars, or AU$92.
"Guys are expected to ring the bells next to the doors. Knocking is considered to be rude," a blogger who goes by the name of Rockit writes.
"Some customers use experience or online reviews to guide them to a particular room but others who are just browsing ride up to the 22nd floor and then take the stairs down until they find a woman to their liking.
"Each room has a unique number and these are used by guys who compare notes online.
"Women come and go all day long as do guys looking for fun. The place never gets uncomfortably crowded but it can get busy at times."
And it's all completely legal.
"Hong Kong is a mongering Mecca!" writes a man who identifies himself as "Brockstar", a travel blogger who writes solely on reviewing sexual experiences around the world.
"Not only do you have (sex workers), but great restaurants, bars, night markets and many sites worth seeing."
Prostitution is legal in Hong Kong, but the region has some unique rules around its sex industry.
By prohibiting brothels and pimps the city has given birth to a culture known as the "one woman, one room" rule.
Soliciting, living off earnings of a sex worker or controlling a woman for the purpose of prostitution is outlawed, but as the South China Morning Post explained, it is only classed as illegal if more than one prostitute is working in the room.
"The Fuji Building may be full of prostitutes, but so long as each individual flat can be shown to be a separate unit from all the rest - that it has its own water connection, electricity meter and no one else can access it - it is above board."
But there are areas of concern, most notably the scourge of human trafficking that's "bubbling beneath the surface of the Hong Kong sex scene".
Police have raided the building before, but not for breaking prostitution laws - they "are breaking immigration laws, as two-way permit holders, or illegal immigrants are living there," a Hong Kong police spokesperson told the South China Morning Post.
"Foreign prostitutes that are soliciting in the street or in bars are also working here on tourist visas, so when we arrest them it's for breaching the condition of their immigration status, not for prostitution."
Gaskell knocked on 15 doors inside the Fuji building and spoke to 10 women, all of whom declined to be named because "I don't think my boss would like it". Not long after, on the 10th floor, she said she was eventually kicked out by a "short man in a black bandana and cropped muscle T".
So the glaring question, Gaskell asks, is who's really in charge?
"How often is it a masquerade, a pretext for pimps and mamasans (female pimps) to control a staggering 18 floors of 'phoenix women' in ever-bustling Causeway Bay?"
Gaskell spoke to a five-year veteran volunteer at Zi Teng - a Hong Kong group which advocates the rights for sex workers - identified only as Lee.
Lee said that many of Hong Kong's 100,00 sex workers align themselves with a group called the Triads - a crime syndicate who will "really do something" if a client doesn't pay.
Relations with authorities are strained and despite improvements, "the mistrust runs back decades".
"I don't know if you can even call it a goal," Lee says of his hopes for decriminalisation.
"It's more like a utopian situation that we hope we can move toward."
For now, men like Brockstar are relishing the current situation.
"In a nutshell, you have over 100 wh*res in one building and it's completely LEGAL!"