Srimad-Bhagavatam: Canto 8 - Chapter 1 - Verse 23
रसामचष्टाङ्घ्रितलेऽथ पादयो-र्महीं महीध्रान्पुरुषस्य जङ्घयो: ।पतत्त्रिणो जानुनि विश्वमूर्ते-रूर्वोर्गणं मारुतमिन्द्रसेन: ॥ २३ ॥
rasām acaṣṭāṅghri-tale ’tha pādayormahīṁ mahīdhrān puruṣasya jaṅghayoḥpatattriṇo jānuni viśva-mūrterūrvor gaṇaṁ mārutam indrasenaḥ
Thereafter, Bali Mahārāja, who was occupying the seat of King Indra, could see the lower planetary systems, such as Rasātala, on the soles of the feet of the Lord’s universal form. He saw on the Lord’s feet the surface of the globe, on the surface of His calves all the mountains, on His knees the various birds, and on His thighs the varieties of air.
The universal situation is described herein in regard to the complete constitution of the Lord’s gigantic universal form. The study of this universal form begins from the sole. Above the soles are the feet, above the feet are the calves, above the calves are the knees, and above the knees are the thighs. Thus the parts of the universal body, one after another, are described herein. The knees are the place of birds, and above that are varieties of air. The birds can fly over the mountains, and above the birds are varieties of air.