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Looking back, it seems Skinevitable that there would be a National Butt Crack Day … and this is the date chosen to ass•ign it to posterior•ity.

A rather recent offshoot of the long-celebrated Moon Day—today marks the 47th anniversary of men walking on the Moon on July 20, 1969 … and like the butt crack, that heavenly body has deep dimples too!

The holiday was initially designed to raise awareness of colon cancer…with last year’s effort marked by “a guerilla marketing campaign that unleashed butt cracks throughout Chicago {home to Mr. Skin}. Butt decals were placed on the backs of bar chairs, barber shop stools and particularly bus seats around the city …

… making it appear that a person—often a comely commuter—was wearing low-waisted jeans that exposed too much butt crack,” reported MediaPost.com. “Coupled with that image was copy stating: ‘No one wants to see this. Except for your doctor.’”

Medically speaking, this rumble seat has a name: the Intergluteal Cleft, which Wikipedia defines as being “known colloquially as the ‘butt crack’ or ‘ass crack.’ It is the groove between the buttocks that runs from just below the sacrum {aka coccyx} to the perineum {or taint}, so named because it forms the visible border between the external rounded protrusions of the gluteus maximus muscles. The intergluteal cleft is located superior to the anus.” And, when done right, it’s definitely superior to the anus!

Indeed, that little fold of flesh has fascinated artists for millennia—from Leonardo da Vinci’s Venus de Milo to Man Ray’s muse Kiki de Montparnasse in his 1924 iconic image Le Violin d’Ingres.

On a less-exalted plane, the Urban Dictionary defines “butt-crack” as ‘the Anal Valley” …

… while using the term in a sentence as requiring teens “to wax their butt-cracks as well as their bushes.” A complaint about ‘crack’ common enough, it appears, to have become a popular Internet meme!

Indeed, ‘reverse cleavage’ seems even more frequent today than frontal flaunting—as evidenced by these lung-layered ladies posing for a street shot unaware of the back•flash.

Why, these days it’s hard to escape it—unlike derrieres from denims—since it’s not only Chicago where gals are showing their “south side.” You see wicked ‘wedge boneeverywhere, from the park to the dumpster …

… and traveling from the shoe store to the food court! Perhaps they got hot cross buns.

Malls seems to be the headquarters for hindquarters—as one Japanese shopper observed in this viral post:

Even the PG aisles of children’s toys and books aren’t immune from the scourge of rumpus delecti.

Clearly, ass-a-holics needn’t confine themselves to ‘where the sun don’t shine’ … since backland destinations have now moved out and about—with the beauteous gluteus gracing town squares from Rome to Scandinavia {though the first looks ‘Greek’ to me}.

And omnipresent cell phones have only added to the ‘Crackish’ epidemic, as users are usually oblivious to everything around them … including cell phone cameras … like these lovely ladies who sat to chat.

It seems the hole world has gone ‘crack’ crazy …

… since today one can see ‘vertical smiles’ for miles!

So what’s a woman to do to prevent her ‘English muffins’ from becoming toast? She can give gawkers ‘the evil eye’ (below left), or laugh off lookers by plumbing the depths of humor (below right).

Or she could avail herself of the latest technology—to combat cell phone fallout—by using the brazen Backtacular Gluteal Cleft Shield. As its actual ad campaign promotes: “Tired of low-riding pants causing unwanted buttock exposure? Want to put an end to bending over only to find that your bum cheeks have been on display to the world? One designer may have come up with a solution for you. The Backtacular Gluteal Cleft Shield, created by entrepreneur Kimberly Brewer, is a stylish cover that prevents any unfortunate butt-crack revelations.”

“Made of hypo-allergenic denim, and decorated with studs and rhinestones for added bling,” the press release notes, “the Shield is applied directly to the skin, and sits above the waistband protecting the wearer’s modesty and morals like a tiny, fashionable nun.”

Let’s hope it doesn’t become a habit {ahem and amen}, since it seems to Sleuth to be the worst idea since ‘titty tape.’ Speaking of which, Hollywood takes a back seat to no one when it comes to trends, so it’s no surprise it’s become the ‘Beautocks’ capital of the world. And Bollywood isn’t far behind … as Indian actress Sofia Hayat “became a star overnight,” according to the Times of India, “at the premiere of The Interpreter—when she was wearing a low-cut designer gown that revealed her butt crack.”

“A London TV presenter and former Bollywood Star contestant upstaged Nicole Kidman at the UK premiere of The Interpreter,” read one report of the May 2005 incident. “Not only was Sofia Hayat not wearing any underwear, she decided to prove it. One can only admire her dedication to the scientific method.” And Sleuth’s to uncovering her bare background {BTW, India is Mr. Skin's second biggest market behind only the States}:

Fast forward to just 9 weeks ago, when the grand Hayat announced she was now a nun … though not as 'fashionable' as the earlier Cleft Shield ... and renounced “all worldly pleasures” with the solemn vow: “I will never have sex again. I had an active sex life,” the newly christened Gaia Mother Sofia confessed. “I had boyfriends. The change happened in July 2015. I asked myself, ‘Why am I not feeling anymore desires?” As for the objects of her desirability, the reborn holy mother im•parts: “This {pointing to her silicone implants} was a fake reality that I was putting on display then. Those who want the physical Sofia will never get it ever again.” So let us pray for one last look back …

With Sofia gone from the seen, the leading lady occupying the rumble seat would seem to be singer AvrilLavigne:

“My butt-crack showing is like my trademark,” Avril admitted. “I wear my pants loose, and lately I’ve been so busy I’ve lost a few pounds. My ass-crack is always showing!” As Mr. Skin wrote in his 2005 Skincyclopedia: “We caught a live performance on Canada’s Much Music {a 2002 awards show in her native land} where her low riders were riding so low that she mooned the audience!” (above left). Well, it is also Moon Day!

Making for a (loose) fitting end to lead-in to a dirty dozen derri•airing damels in dis•dress.

In keeping with the sanctity of National Butt Crack Day, we’ve limited our choices to un-posed, can•did ‘crack•cidents’—since otherwise the stars were just doing it for provocative publicity {and were thus just ass•king for it!}.

And no thongs or ‘whale tail’ exposures, since those have their own holiday in March {not to be confused with the thong sandal celebration every October}.

So, without further ado, let’s take a stroll down Happy Valley … {note the brown headers for eacheye•ful}:

BRITNEY SPEARS

CHRISTINA AGUILERA

HEIDI KLUM

KATHERINE HEIGL

JESSICA ALBA

Whether wise•cracking at the beach above … or shopping at W(hole) Foods below ...

… fittingly, her tramp-stamp tattoo is of a butt•erfly!

MICHELLE TRACHTENBERG

Buffy’s little sister hails a cab with her cab•oose

KATE BECKINSALE

Rather than slaying vampires, she’ll reprise her role as one in October’s upcoming fifth Underworld film, directed again by her hands-on hubby Len Wiseman {aka lucky man}

MARISA MILLER

The Sports Illustrated Swimsuit fixture was picked as MVP of the Celebrity Beach Bowl in 2010 when cheeky Tom Arnold decided to forego her flag around the waist and tackle her tight leggings instead: “And by the looks of things,” quipped the Daily Mail, “it would seem that Miss Miller doesn’t wear Victoria’s Secret underwear outside of work” (above). “All of a sudden, this big sweaty guy was on top of me,” Marisa moaned afterwards. But not before reversing the curse to come out on top … and bottom (below)!

MICHELLE RODRIGUEZ

Had a small black bikini on beneath her pants on 5/27/11 …

MAITLAND WARD

KIM KARDASHIAN

She’s brought back the ‘Bosom of the Pants’ to popularity, so it’s no surprise the future reality star’s first major modeling gig was the 2007 crack attack above.

Or that she previewed an episode of Keeping Up with the Kardashians with this outtake trying on tight jeans.

Nowadays, whether braving the surf or going through airport security, Kim’s culo cleavage remains her best ass•et!

But the undisputed ‘Queen of Crack’has to be Ben Affleck’s soon-to-be ex and our Number One butt barer…

JENNIFER GARNER

Here’s a quartet collage of Jen’s at least half dozen never-fail-ius Ali-ass appearances

Let’s END with the last of our Top FORTYdualshot greatest hits ...

… with nary a crack backblock in sight!