Sage Vyasa was meditating in the Himalayas when Lord Brahma appeared before him and asked him to write the  Mahabharatham for the benefit of all.Sage Vyasa said  that he could not compose the verse and write it down too and therefore asked for the help of someone wise, who would write down as he sang.Brahma then asked sage Vyasa to pray to Lord  Ganesha to help him.

 Ganesha told Vyasa that he had enormous work to do and hence he would write it down,only if Vyasa was quick in singing.For his turn Sage Vyasa requested that Ganesha should understand the meaning of the verse and only then put it down in writing.

Then Vyasa started 
singing.Whenever he felt Ganesha was writing too fast for him,he would come with a difficult verse,thus by the time Ganesha could decipher the meaning,he would  have composed several more in verses in his mind.

In this way,Vyasa completed the Bharatham 60 lakhs granthams.This comprises of the Kandapuranam,
Ramayanam, Savithri Puranam, Nala Puranam and so on.The Lord Ganesha  wrote on Mount Meru using his tusk.

Of these 60 lakh granthams,30 lakhs are supposed to be in the Deva Logam,15 lakhs in Yatcha Logam and  14 lakhs in Asura Logam.Remaining 1 lakh granthams is  assumed to be on the earth. Sage Vyasa is supposed to
have sung 8,800 granthams,which were difficult for Ganesha to decipher.

He has himself stated that Ganesha, Sage Vyasa himself and his Sugan may be the only ones who know the meaning of those 8,800 granthams.The Mahabharatam is considered the fifth Veda.