Dānishnāma-ye Imām Mahdī bar Pāya-ye Qurʾān, Ḥadith wa Tārīkh (Persian: دانشنامهٔ امام مهدی, lit. the encyclopedia of Imam Mahdi developed upon Qur'an, hadith and history) is a 10-volume book in Persian that discusses topics related to Mahdism based on verifiable Islamic scriptures. It is written by Muhammad Muhammadi Rayshahri and assisted by Sayyid Muhammad Kazim Tabatabayi, Mahmud Karimiyan, Muhammad Rida Sajjadi Talab, Sayyid Mahmud Tabatabayi Nizhad and Sayyid Ruhullah Sayyid Tabayi. It is translated to Arabic by 'Abd al-Hadi Mas'udi and printed by Dar al-Hadith Publications.
Signs of the reappearance of Imam Mahdi (a), as narrated, fall into two categories of mutable and immutable. An abundant number of narrated traditions report that five of these signs are immutable in nature. These are: the uprising of the Sufyani, the uprising of the Yamani, the celestial proclamation that shall come from the sky, the killing of Nafs al-Zakiya, and the sinking of the earth at Baydah. The mutable ones include: Persistent and successive rains at the threshold of the Imam's reappearance, irregular instances of solar and lunar eclipses prior to the Imam's return, widespread deaths, earthquakes, wars and pervasive unrest and turmoil etc. Neither of the immutable signs have come to pass, nor can there be any definitive discussion regarding the mutable ones.
The reappearance of Imam Mahdi (a) constitutes the single most illustrious incident within the history of humanity. As such, and without even an iota of doubt, there exist numerous signs for this great event mentioned within the narrated traditions of the Infallibles (a).
According to the narrated discourse of the Infallibles (a), events that are counted as signs of Imam Mahdi's reappearance have two main attributes. Firstly, they represent incidents that are to occur at the threshold of Imam Mahdi's reappearance or concurrent to his arrival or immediately after the Imam's return. Secondly, they must usher in glad-tidings about his reappearance and the imminence of his global uprising.
From a chronological point of view, the signs of Imam Mahdi's reappearance fall within four types and categories. These are as follows: Signs that shall occur a good time before the Imam's reappearance, Signs that shall occur immediately before the Imam's reappearance and at the "threshold of his return", Signs that shall occur concurrent to the reappearance of Imam Mahdi (a), Signs that shall occur immediately after the reappearance of Imam Mahdi (a) but prior to the Imam's uprising.