The story of the descent of Ganga is very well known. King Sagara's 60,000 sons were burnt to ashes by their own sin of insulting Lord Kapila, and to offer redemption to them, their nephew's grandson Bhagiratha brought down the Ganga to the earth.
But my question is more fundamental. How come Sagara had 60,000 children with Sumati (even though she was Garuda's sister she couldn't have had such magical powers)? And that too when his other wife Keshini had only one son Asamanjasa?