The knees of the universal form are the planetary system of the name Sutala, and the two thighs are the Vitala and Atala planetary systems. The hips are Mahītala, and outer space is the depression of His navel.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
द्वे जानुनी सुतलं विश्वमूर्ते- रूरुद्वयं वितलं चातलं च । महीतलं तज्जघनं महीपते नभस्तलं नाभिसरो गृणन्ति
dve jānunī sutalaṃ viśvamūrte- rūrudvayaṃ vitalaṃ cātalaṃ ca | mahītalaṃ tajjaghanaṃ mahīpate nabhastalaṃ nābhisaro gṛṇanti
SB 2.1.27