"They're digging at the wrong place!"

From Indiana Jones:

[the old man reveals writing on the back of the medallion, which states that part of the staff must be removed]
Indiana: Balloq's medallion only had writing on one side? You sure about that?
Sallah: Positive!
Indiana: Balloq's staff is too long.
Indiana, Sallah: They're digging in the wrong place!

well, in film 3 of the series, they "found" the Holy Grail in...Petra. (not really Petra, but that was the place where they filmed)

But today's Atlas Obscura has a post about the real place where the Holy Grail is located...
well, not really the Holy Grail, but a replica of the replica...

Tucked high in the Pyrenees, San Juan de la Peña ranks among the most important religious sites in the Aragon region of Spain thanks to centuries of royal pilgrimages, paired with legendary tales linking it with Christianity's most coveted treasure: the Holy Grail. (according to one story)  a Roman soldier acquired what is claimed to be the much sought-after Holy Grail at some point during the 3rd century CE. After transporting this most coveted relic back to his home at Huesca, a Moorish invasion caused the cup to be transferred to San Juan de la Peña where it would be safe from marauders. Little did they anticipate that Aragonese King Martino V would abscond with it to his palace in 1399...

source: Spanisculture 

more information here...