Śatapatha Brāhmaṇa ("of a hundred paths") 7.5.1.2

7.5.1.2

tasya yadadharaṃ kapālam | ayaṃ sa lokastatpratiṣṭhitamiva bhavati pratiṣṭhita iva hyayaṃ loko 'tha yaduttaraṃ sā dyaustadbyavagṛhītāntamiva bhavati vyavagṛhītānteva hi dyauratha yadantarā tadantarikṣaṃ sa eṣa ima eva lokā imānevaitallokānupadadhāti

That lower shell of it is this (terrestrial) world; it is, as it were, fixed; for fixed, as it were, is this (earth-)world. And that upper shell of it is yonder sky; it has its ends, as it were, bent down; for yonder sky has its ends, as it were, bent down. And what is between (the shells) is the air;--that (tortoise) thus is these worlds: it is these worlds he thus lays down (to form part of the altar).