SB 2.7.1

brahmovāca
yatrodyataḥ kṣiti-taloddharaṇāya bibhrat
 krauḍīṁ tanuṁ sakala-yajña-mayīm anantaḥ
antar-mahārṇava upāgatam ādi-daityaṁ
 taṁ daṁṣṭrayādrim iva vajra-dharo dadāra
Word for word: 
brahmā uvāca — Lord Brahmā said; yatra — at that time (when); udyataḥ — attempted; kṣiti-tala — the planet earth; uddharaṇāya — for the matter of lifting; bibhrat — assumed; krauḍīm — pastimes; tanum — form; sakala — total; yajña-mayīm — all-inclusive sacrifices; anantaḥ — the Unlimited; antar — within the universe; mahā-arṇave — the great Garbha Ocean; upāgatam — having arrived at; ādi — the first; daityam — demon; tam — him; daṁṣṭrayā — by the tusk; adrim — the flying mountains; iva — like; vajra-dharaḥ — the controller of the thunderbolts; dadāra — pierced.
Translation: 
Lord Brahmā said: When the unlimitedly powerful Lord assumed the form of a boar as a pastime, just to lift the planet earth, which was drowned in the great ocean of the universe called the Garbhodaka, the first demon [Hiraṇyākṣa] appeared, and the Lord pierced him with His tusk.
Purport: 

Since the beginning of creation, the demons and the demigods, or the Vaiṣṇavas, are always the two classes of living beings to dominate the planets of the universes. Lord Brahmā is the first demigod, and Hiraṇyākṣa is the first demon in this universe. Only under certain conditions do the planets float as weightless balls in the air, and as soon as these conditions are disturbed, the planets may fall down in the Garbhodaka Ocean, which covers half the universe. The other half is the spherical dome within which the innumerable planetary systems exist. The floating of the planets in the weightless air is due to the inner constitution of the globes, and the modernized drilling of the earth to exploit oil from within is a sort of disturbance by the modern demons and can result in a greatly harmful reaction to the floating condition of the earth. A similar disturbance was created formerly by the demons headed by Hiraṇyākṣa (the great exploiter of the gold rush), and the earth was detached from its weightless condition and fell down into the Garbhodaka Ocean. The Lord, as maintainer of the whole creation of the material world, therefore assumed the gigantic form of a boar with a proportionate snout and picked up the earth from within the water of Garbhodaka. Śrī Jayadeva Gosvāmī, the great Vaiṣṇava poet, sang as follows:

vasati daśana-śikhare dharaṇī tava lagnā
 śaśini kalaṅka-kaleva nimagnā
keśava dhṛta-śūkara-rūpa
 jaya jagadīśa hare

“O Keśava! O Supreme Lord who have assumed the form of a boar! O Lord! The planet earth rested on Your tusks, and it appeared like the moon engraved with spots.”

Such is the symptom of an incarnation of the Lord. The incarnation of the Lord is not the concocted idea of fanciful men who create an incarnation out of imagination. The incarnation of the Lord appears under certain extraordinary circumstances like the above-mentioned occasion, and the incarnation performs a task which is not even imaginable by the tiny brain of mankind. The modern creators of the many cheap incarnations may take note of the factual incarnation of God as the gigantic boar with a suitable snout to carry the planet earth.

When the Lord appeared to pick up the earth, the demon of the name Hiraṇyākṣa tried to create a disturbance in the methodical functions of the Lord, and therefore he was killed by being pierced by the Lord’s tusk. According to Śrīla Jīva Gosvāmī, the demon Hiraṇyākṣa was killed by the hand of the Lord. Therefore his version is that after being killed by the hand of the Lord, the demon was pierced by the tusk. Śrīla Viśvanātha Cakravartī Ṭhākura confirms this version.