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?
King Sagara who being childless propitiated the saint Bhrigu by penances extending over a hundred years. The saint, pleased with Sāgara's worship said that from one wife of his, he will have a son from whom his heir and maintainer of lineage and from other, 60,000 sons will be born, but he didn't specify who among Kesini or Sumati will get what.
Later, in Ramayana (XXXIX), Viswamitra says to Rama:
Then Sumati, the younger, gave
Birth to a gourd, O hero brave,
Whose rind, when burst and cleft in two,
Gave sixty thousand babes to view.
All these with care the nurses laid
In jars of oil; and there they stayed,
Till, youthful age and strength complete,
Forth speeding from each dark retreat,
All peers in valour, years, and might,
The sixty thousand came to light.
It's clear that Sumati didn't give birth to 60,000 sons through natural process. As it happened with Gandhari, Sumati's sons had a miraculous birth.
As discussed here in Vishnu Purana about Sagara's 60000 sons.
SUMATI the daughter of Kaśyapa, and Kesiní the daughter of Rája Viderbha, were the two wives of Sagara 1. Being without progeny, the king solicited the aid of the sage Aurva with great earnestness, and the Muni pronounced this boon, that one wife should bear one son, the upholder of his race, and the other should give birth to sixty thousand sons; and he left it to them to make their election. Kesiní chose to have the single son; Sumati the multitude: and it came to pass in a short time that the former bore Asamanjas 2, a prince through whom the dynasty continued; and the daughter of Vinatá (Sumati) had sixty thousand sons.
From Another Source
According to history of Ramayan, Ikshvaku dynasty’s king Sagar had two queens – Keshini and Sumati. After remaining childless for several years, King and both Queens went to Himalay to perform strict penance and abide by the Vedic rituals of obtaining child through unborn process. Maharishi Bhrigu blessed them that Sumati will give birth to 60,000 haughty sons and Keshini will give birth to a son whose son will continue the lineage. As years passed, Sumati gave birth to pumpkin sized Garbh pind. Raja Sagar was not amused, he was about to throw it away, he heard Akashvani that “the Garbh Pind has 60,000 fertile seeds. You need to keep each seed in a ghee filled 60,000 separate pots.” Raja Sagar obeyed the Akashvani thanking Bhagwan Shiv for his blessings. In due period, the pots gave birth to 60,000 sons of Sumati. Later King Sagar thought of performing Ashwamedh Yagya, he asked all his 60,000 sons to protect the horse and make the Yagya successful. While that Ashwamedh horse is being moved around provinces it reached the south-easterly zone, and at seashore Indra abducted that horse and hid it underneath the earth. Raja Sagar then ordered his escorting sons that horse to dig up that place to locate the ritual horse. Then those princes unremittingly dug the place to oceanic depths until they reached Shrihari reposing there in the form of Sage Kapil. When the 60,000 sons found the horse at Kapil dev’s ashram, they thought he had stolen it. When they prepared to attack the meditating rishi, Kapil opened his eyes, fire emanated, and they were immediately burned to ashes.
