Yes, it is. Something strange seems to be going on there, but the amount of effort that had been put into the whole thing is phenomenal.
Where’s van Gogh’s Portrait of Dr. Gachet?
It’s more of a secret than a mystery, but IIRC, the most famous Chanel perfume (no. 5? ) has a closely held, top-secret formula. Competitors have even put it under electron microscopes, and can’t figure out what they’re looking at.
My favorite theory: these old ladies are spraying themselves with weasel vomit.
What Jesus was writing in the dirt when He told the crowd that the first one among them who is without sin should cast the first stone at the adulterous woman.
The OP said Dark Matter doesn’t count.
Any of these count?
And lots of it. 
I know Cervaise is disappointed about not being able to make genuine oysters Rockefeller, but the Waldorf-Astoria would be glad to send him the recipe for their red velvet cake for only $200.
It may be a few more years, but this is being worked. I’ve seen DepthX up close and personal and it is one seriously cool piece of kit.
This book might be of interest to the OP. I’m told that very little of it is outdated. Encyclopedia of Ignorance
Disappearances of the type mentioned in various posts are kind of a gray area – there are some indications of where to start looking, but the evidence may simply no longer exist (which means that nobody has a chance to be “looking right at it” any more).
The one that nobody mentioned yet that comes to mind is the fate of the Amber Room (which presumably required a fair number of people to move to wherever it ended up).
Thanks for sharing that link. I’d never heard of this manuscript before and now I’m fascinated, too.
What happened to to the Neanderthals? Did they die out entirely and, if they did, why? Or were they able to interbreed with Cro-Magnon and pass some of their genetic material down to us? Is that child skeleton found in Porugal a true Neanderthal-Cro Mag hybrid? Neandertal-Cro-Magnon Hybrid? - Archaeology Magazine Archive
Ditto, double ditto, and triple ditto on the Voynich manuscript, and also:
Who was the ‘dark lady’ and the unnamed man in Shakespeare’s sonnets?
Heyyyyy…this topic rings a bell. 
There’s an equally famous case in soccer - Geoff Hurst’s “goal” in the 1966 World Cup Final. Everybody in England (and Scotland and Wales and probably elsewhere, much to their annoyance) has seen the footage hundreds of times. And though I’m English I’m pretty sure that it wasn’t actually a goal.
The identity of Beethoven’s Immortal Beloved?
Yea but, If the source code of Windows is more valuable than the Holy Grail and it seems like every two-bit hacker can find a security flaw, why hasn’t the code been cracked?
That there is a Damn Good Question! The man can write, and writers—WRITE.
Define “up close and personal”. Are you actually allowed to breathe in the same room as it? I would have thought that, with as thorough as they’re being on that thing to prevent the possibility of contamination, that they wouldn’t let any organic material in its presence, and do all the work with waldoes.
Jackmannii writes:
> I know Cervaise is disappointed about not being able to make genuine oysters
> Rockefeller, but the Waldorf-Astoria would be glad to send him the recipe for
> their red velvet cake for only $200.
You got the recipe for only $200? I got cheated. They charged me $500 for it.