Story of Nether Worlds as per Vishnu Purana. Vishnu Purana explains about Nether Worlds in its Chapter Five.
Earth’s total height is 70,000 Yojans. In her depth, are situated seven nether worlds with a depth of 10,000 Yojans each respectively. Names of these seven nether worlds are Atal, Vital, Nital, Gamastiman, Mahatal, Sutal and Patal. These nether worlds have beautiful palaces, and have lands which are white, black, red and yellow in colour with gravel, rocky and golden soil. Scores of races including Danav, Daitya, Yaksha and Nagas live there.
Once upon a time, Narada visited these nether worlds. When he returned to heaven, he told the people there that the nether world was more beautiful than heaven. He told them that ornaments of the Nagas contained precious gemstones and the womenfolk of Daitya and Danavas were an attraction even for the gods. The sun was illuminating in the nether worlds but not scorching. Similarly, the Moon only shone there but did not cause bitter cold.
Beneath the nether worlds is situated an idolised form of Lord Vishnu. It is known as Shesh. Even the demons and giants cannot describe His virtues. He has thousands of hoods each carrying a bright gemstone that illuminates the azimuths. In His presence, the demons lose their strength. His eyes are red and He wears an earring, crown, garland and blue length of cloth. Bearing the flow of Ganges, He stays like the Kailash Mountain. At the end of the Kalpa, poison emanates from His thousands of hoods and destroys all the three worlds taking the form of Rudra named Sankarshan. Nobody can find the ends of His virtues; hence He is known as Anant.