According to authentic narrations it is related from ‘Ali ibn Yaqtin that Mansur (the `Abbasid caliph) ordered him to dig a well in the palace of a pious man. Yaqtin began to dig the well and continued for a considerable period of time. In the due course, Mansur died. But there was still no water in this well. When al-Mahdi learnt of this he said that he would continue digging until water is struck. Even if the public treasury is exhausted in this venture.
Hence Yaqtin appointed his brother Abu Musa for this well. He continued the digging to such an extent that a hole appeared at the bottom of this well. A whiff of breeze came through the hole. The diggers were terrified and they came and informed Abu Musa. He came to the well and asked to be lowered into the well. The well was forty hands wide. The people lowered Abu Musa in a canopy tied with ropes. When he reached the bottom he sensed a great peril from this hole. He heard the rumbling of wind below it. Then he ordered the widening of this hole and lowered two people to investigate the matter. The two of them remained inside for a long time and at last tugged their ropes to pulled back. When they were pulled to the surface they related astounding things. “We saw men who have turned to stones. All their belongings and houses etc have turned into stones. There men and women are wearing clothes. Some of them are sitting and some lying on their sides. Some of them are resting on pillows. When we touched their clothes they turned to dust and flew away in the breeze. Their houses retain their original structure.
Abu Musa wrote to al-Mahdi about it and all the court scholars were astonished. But no one could think of a rational explanation. At last al-Mahdi, the ‘Abbasid wrote a letter to Madinah and requested Imam Musa al-Ka¨im to come and explain the enigma. Imam traveled to Iraq. al-Mahdi related the incident to Imam. Imam wept profusely and said that those were the remains of the people of ‘ad. The Almighty sent His wrath upon them and they sunk below the earth along with their houses. And they were the people of Ahqaf. “What is Ahqaf?” asked al-Mahdi. “Sand,” replied Imam.
Reference: Hayat Al-Qulub, Vol. 1, Stories of the Prophets