Hazrat Mahdi (as) will unearth the ark of the covenant

The reason he is called Hazrat Mahdi (as) is that he shows the way to a hidden thing. He will bring forth the Ark (Ark of the Covenant) in a place known as Antioch. (As-Suyuti, Al-Hawi li al-Fatawa, II. 82)

He [Hazrat Mahdi (as)] will bring forth the Ark of the Covenant from the cave of Antioch. (Nuaim Ibn Hammad, Kitab Al-Fitan)

"Hazrat Mahdi (as) will bring the Ark of the Covenant out of the cave of Antioch." (Ibn Hajar al-Haythami, Al-Qawl al-Mukhtasar fi `Alamat al-Mahdi al-Muntadhar, 54)