When Priyavrata drove his chariot behind the sun, the rims of his chariot wheels created impressions that later became seven oceans, dividing the planetary system known as Bhū-maṇḍala into seven islands.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
ये वा उ ह तद्रथचरणनेमिकृतपरिखातास्ते सप्तसिन्धव आसन् यत एव कृताः सप्त भुवो द्वीपाः
ye vā u ha tadrathacaraṇanemikṛtaparikhātāste saptasindhava āsan yata eva kṛtāḥ sapta bhuvo dvīpāḥ
SB 5.1.31