Bhageerath Jee brought Gangaa on Prithvi to give Mukti to his 60,000 great grandfathers.

So Raajaa Bhageerath was in the lineage of Raajaa Sagar. Sagar --> 60,000 sons and Asmanjas --> Anshumaan --> Dileep --> Bhageerath. Bhageerath did a very hard Tap and pleased Gangaa Jee. She gave Darshan to him and asked him to ask for any Var. Bhageerath explained everything and politely requested her to come to Mrityu Lok. Gangaa said - "Yes, I can come, but there should be somebody who can bear my force of falling down. Otherwise I will break the Prithvi and and go to Rasaatal. Besides I will not go to Prithvi, only because that people commit sins and wash their sins in my waters, then where will I wash my sins? So think carefully and tell me."

Bhageerath said - "Hey Maa, Who have no desire of Lok Parlok, wealth, women and sons, and who are always busy in others welfare they will destroy your sins with their touch. And Rudra Dev will bear your force." After saying this to Gangaa, Bhageerath pleased Bhagvaan Shankar with his Tap. Then Shiv Jee bore Gangaa on His head. Gangaa thought to play a little game with Him, so she fell with a fierce force, but Shiv Jee was ready for that, so He also played a little game with her. He spread his hair so complexly that Gangaa couldn't find the way to come out.

Bhageerath got worried, he again prayed Shiv Jee to release Gangaa Jee. Then He unlocked his one flock of hair and a thin stream of Gangaa came out of His head. First Bhageerath took her to Kapil Muni's Aashram where his great grandfathers ashes were lying. As Gangaajal (Gangaa waters) touched it all his great grandfathers got Mukti. And Bhageerath and all Pitar went to Swarg. Kapil Muni's Aashram was on Gangaa Saagar Sangam. 

My Note
Gangaa Saagar place is near Calcutta. This is an island under the sea, but it comes out once a year for three days at the time of Makar Sankraanti. People go there and take a holy dip and have a religious fair at that time. Government always tries his best to make sure that nobody remains on that island after the prescribed time. Because after that it again merges in the sea.