Vyasa first recited his Mahabharata to Ganesha. He also recited it to his disciples, including Vaishampayana, who repeated what he heard to Janamejaya.
Jaimini, another disciple of Vyasa, wrote his Mahabharata after meeting some birds who were present at Kurukshetra. An arrow had struck their mother flying overhead, and her eggs landed on the blood-soaked ground. Then a war-elephant's bell fell, covering the eggs and protecting them. The birds hatched and heard the war from inside the bell.
Alas, most of Jaimini's Mahabharata is lost, but there are more Mahabharatas.
Now you've reached the end of this one.
Inspired by: Jaya: An Illustrated Retelling of the Mahabharataby Devdutt Pattanaik.
Notes: This story is from the Author's Note: What Ganesha Wrote. Vyasa also famously recited the stories to Ganesha, and he also told the stories to his son, Shuka, who told them to Parikshit, Janamejaya's father.