Srimad Bhagvata Mahapurana: Book 3: Chapter 13: Verses 18-30
As he was thus reflecting, a tiny boar, just as big as a thumb, emerged all of a sudden from one of his nostrils, O sinless Vidura. While Brahma stood looking on, the boar in the air instantly grew to the size of an elephant, O Vidura (descendant of Bharata)l This was really most wonderful. Beholding that (strange) form of a boar, Brahma with Marici and other Brahmanas, Sanaka and his three brothers and Swayambhuva Manu speculated about it in various ways: "Who can be this weird creature standing in the guise of a boar ? Oh, how strange that it came out of my nostrils! Even though (at first sight) it looked only as big as the upper part of a thumb, in an instant it grew to the size of a huge rock. Might it be Lord Visnu Himself who is exercising my mind ?" While Brahma was thus pondering along with his sons, the Lord presiding over sacrifices (for the boar was none other than He) who now looked like a huge mountain, roared aloud. The all-powerful Sri Had delighted Brahma and those foremost Brahmanas by His loud roar, which made the quarters resound. Hearing the roar of the Lord disguised as a boar, which removed their perplexity, the sages, who all belonged to the Janaloka, Tapoloka or Satyaloka, began to extol Him through the holy Mantras of the three Vedas. The Lord, whose form has been glorified in the Vedas, took the tribute paid to His virtues by those holy men as Veda itself. He gave a loud roar once more and, sporting like a lordly elephant, entered the waters in the interest of the gods. The divine Boar (the Saviour of the earth), who was possessed of a steel-like frame and whose skin had a thick coat of stiff bristles, first sprang into the air, erecting His tail and shaking His mane, and tore the clouds with His hoofs. He shone most splendid at that time with His white tusks, shedding a lustre wherever He cast His glance. Having disguised Himself as a boar, even though He was no other than Lord Narayana, representing all the sacrifices in His own person, He went smelling about with His nose in search of the earth; and even though possessed of fearful tusks, He regarded with a gentle look the Brahmanas (Marici and others) who were extolling Him, and dived into the water. With its bowels torn open by the headlong plunge taken by His body, which resembled a mountain of adamant, the ocean thundered aloud and, tossing up its high waves like so many arms, cried as it were in distress: "Protect me, O Lord of sacrifices!" Cleaving the waters with His hoofs, which were sharp as arrows, the Lord,.who represents all the sacrifices in His person, then reached the other end of that fathomless ocean and discovered in the depths of it the terrestrial globe, which is the abode of all living beings; and which the Lord had absorbed into His body while about to retire at the end of the previous Kalpa.