Several days ago I commented on political Mahdism in North Africa and "al-Andalus" (once-Islamic Spain), leading with an observation that Muslim messianism is an integral substratum of all Islamic (Sunni as well as Shi`i) civilization--not reducible to (or dismissable as) a belief that is merely episodic and ephemeral.
Today I wish to adduce two further examples of the very deep roots which Mahdism has in Islamdom.  One new case, from Twelver Shi`i Iran, consists of an unidentified ayatollah on one of the state-run TV channels there babbling on breathlessly at some length about how the real Torah (Hebrew Scriptures) and "Injil" ("Gospel," what Muslims call the New Testament) are currently in the possession of the 12th Imam al-Mahdi, probably somewhere in Kufa or Najaf--and he has these divine books wrapped up in a goatskin for safe-keeping, until such time as he finally reveals himself (whereupon said holy books will serve as literal proof texts of his divine legitimacy for Jews, Christians and Muslims--or so the theory goes).
A somewhat different take on the Mahdi proffering his bona fides exists in the Sunni tradition, as articulated most recently (and cogently) by Adnan Oktar, a.k.a. "Harun Yahya" ("Aaron John"), the famous Turkish sect leader whose followers fancy him the Mahdi.  The Sunni Mahdi has a bit more class about stowing recorded divine revelations than the Shi`i one, however--rather than rank animal skins, he will use the Ark of the Covenant, which he will (re)discover near Lake Tiberias (the Sea of Galilee) upon his return, according to several Sunni hadiths (alleged extra-Qur'anic sayings of Muhammad).  Oktar glosses these hadiths to mean that the Mahdi could find the Ark in Antioch, Mecca, Jerusalem or even Istanbul (although only the first of those is "near" the relevant body of water). 

The Mahdi, disguised as Dr. Henry Jones, Jr., looking for
the Ark. 

Observations
1) This trope that the Mahdi will reveal the true teachings of Judaism and Christianity--which will no doubt look remarkably like those of Islam--stems from the core Islamic doctrine that both the other major monotheistic faiths are "corrupted" and need purifying and, indeed, supplanting.
2) Asserting the 12th Imam al-Mahdi of Iranian and Iraqi Shi`ism is already present on Earth is a major development in that branch of Islam; heretofore, for the previous eleven centuries since he went missing, Twelver Shi`is have been limited merely to looking forward to his advent and only on rare and remarkable occasions (such as with the claimed eschatological inception of the Safavid Dynasty in the early 16th century) has this orthodoxy been challenged with allegations that the waiting is over.  Other examples of the claim to realized Mahdism in Iran/Iraq, as with the Bah`ai faith starting in the 19th century, have resulted in marginalization and persecution.  The eschatological escape hatch (from heresy) for this ayatollah would seem to be that he never claims any particular person in Kufa or Najaf is the Mahdi--but only makes vague allusions to an unspecified Mahdi being there (in a David Nivenish sense, one suspects).
3) Note that the Twelver Imam al-Mahdi lurks about Kufa or Najaf, Iraq.  Certain Shi`i  traditions hold that the once-and-future Mahdiyah (Mahdist state) will be headquartered in one of those two cities--another reason for Iran to have as its eventual goal the incorporation of at least those parts of Iraq into its ambit. 
4) I would wager that Adnan Oktar adds the possibility of Istanbul being a resting place of that particular ancient "radio for talking to God" just in case he ever decides to openly claim the Mahdiship, and/or to keep the air of Mahdist legitimacy hovering about the old Ottoman imperial capital. (Who knows, maybe President Erdoğan will beat Oktar to it--although I'd expect a caliphal claim from him first.) 
5) On a more archaeological note: perhaps it is not out of the realm of possibility that some shred of historical memory survived in the Sunni hadiths regarding the Ark, and that rather than being buried under the Temple Mount, sequestered in the Chapel of the Ark in Axum (Ethiopia), or lost in a huge government warehouse the Lost Ark of the Covenant is hidden somewhere in northern Israel?  Any archaeologists who wish to discuss this idea, please contact me!

Even the Twelfth Imam will need help finding it in that mess.  He should solicit the assistance of top men. Top. Men.