The writers of Eternal Sunshine of the Spotless Mind, Grosse Pointe Blank and Sherlock Holmes have spent the last ten years searching the Mojave desert for Rocky II, a fake boulder put there by Ed Ruscha:

A BBC crew filmed Ruscha during its creation for a 1980 documentary, which also captured him depositing the work somewhere in the desert, where it has apparently remained ever since, indistinguishable from all the other rocks around it. … Rocky II is so mysterious it neither appears on the call for information about missing artworks listed on the artist’s website, nor in the catalogue listing all his known works – almost as if its existence has been intentionally obscured.

I love this. I kind of hope they never find it, but also that they never, ever stop looking.