If finding the La Bodega Negra restaurant isn't enough of a challenge, you then have to brave walking inside.  Located on a street in London's Soho district, the historic centre of the city's adult entertainment industry, the entrance is designed to look like a sex shop.  Beside the recessed black door, neon lights proclaim "adult video", "peep show" and "girls, girls, girls".  In no way would a passerby guess that the venue was a Mexican restaurant - there is no restaurant signage whatsoever.  Instead you pass into a gloomy entrance area, and then walk down a dark stairway, until you then finally see the basement restaurant appear.  First opening in 2012, La Bodega Negra has been at the forefront of a growing global trend - the rebirth of the "speakeasy", bars and restaurants that hide their location.  The word speakeasy was first coined in the US during the prohibition era, when the sale of alcohol was generally illegal from 1920 to 1933.  To avoid police raids and prosecution, bars that sold alcohol would keep a very low profile. And their customers were told to speak quietly (speakeasy) about them.  Fast-forward to today, and with alcohol legal and freely available in most countries, why do some venues want to hide themselves away? And in doing so, how do they, at the same time, go about attracting customers?  Will Ricker, owner of La Bodega Negra, admits that "there is a fine line between keeping something exclusive and generating revenues".  Yet the 44-year-old adds that "you've got to stand out" in London's competitive restaurant market, and he says that the sex shop frontage has certainly got people talking.  Mr Ricker has also been very successful in attracting celebrities to La Bodega Negra, with rock band U2 hosting a Halloween party in the restaurant a few years ago, and former footballer David Beckham hiring the venue for a party.  Such celebrity approval has helped La Bodega Negra become a fashionable place to be seen, and it is packed out most evenings with people happy to pay its premium prices.  Berlin cocktail bar Beckett's Kopf is another modern day speakeasy.  Located on a quiet side street in the hipster neighbourhood of Prenzlauer Berg, rather than have its name outside, the owners have simply hung a picture of Irish poet Samuel Beckett (from whom the bar takes its name) in the window.  "We wanted to create an atmosphere of discovery, and bring back the curiosity you have as a child," says Oliver Ebert, 42, who opened the venue with his wife Christina in 2004.  With no name on display, and reliant solely upon word of mouth to build up business, Mr Ebert admits that it took three or four years to establish a strong customer base.  Now as many as 120 drinkers pour into Beckett's Kopf on weekend evenings.  For New York-based marketing and branding expert Allen Adamson modern day speakeasies are all about selling exclusivity, which is increasingly desirable to higher-end consumers.  "Exclusivity still drives desire and premium-ness," he says. "Part of effective luxury marketing is some sort of scarcity, or the need to dig deeper to find the story."  Robert Jones from London-based brand consultancy Wolff Olins agrees, saying: "There's a huge cachet in rarity, obscurity, mystique. Inaccessible means desirable."  In Toronto, Canada, the Libertine Speak is another speakeasy bar which thanks in no small part to its exclusivity is regularly packed out.  Instead of having the bar's name outside the venue in the city's West End district, there is a neon sign offering palm readings.  "From the outside it looks like a dive, or a place to get your fortune read," says Philipp Dumet, who bought the bar last year.  "[The speakeasy model] caters to our target clientele - the influencers, the tastemakers, the sort of quote, unquote cool kids of the west end, who operate really solely through the internet."  To engage with his customers, and promote the business, Mr Dumet, 29, extensively uses social media services Twitter, Instagram and Facebook.  Mr Adamson and Mr Jones both agree that the growth of social media in recent years has transformed a company's ability to create positive word of mouth, making it much easier, and cheaper, for would-be speakeasies to built up an exclusive customer base.  "Social media is the rocket fuel that has changed the marketing game," says Mr Adamson, while Mr Jones says word of mouth has been "turbo-powered by social media".  Meanwhile, fellow branding expert Rebecca Battman, says that word-of-mouth marketing "is now at its most powerful and effective" and affordable, thanks to social media.  However, Mr Adamson cautions that whatever the buzz a hidden bar or restaurant manages to create, it cannot forget the basic need to offer excellent service, drinks and food.  "In today's world, nothing will undo you more than a product that's disappointing," he says.
A look at the growing number of bars and restaurants which actively try to hide their location, and how they are able to attract customers while managing to stay under the mainstream radar.