Valmiki Ramayana Bala Kanda Sarga 44 Shloka 1

स: राजा that king, तदा then, गङ्गया by Ganga, अनुगत: followed by, सागरम् ocean, गत्वा having gone, यत्र where, ते they (sons of Sagara), भस्मसात् into ashes, कृता: had been made, भूमे: earth's, तलम् lower regions, प्रविवेश entered.

Thereupon that king followed by Ganga reached the ocean and entered the lower regions of the earth where the sons of Sagara were found reduced to ashes. King Bhagiratha performs funeral rites of his ancestors.