Mahanarayana Upanishad, a part of taittiriya arankya describes about it.
bhūmirdhenurdharaṇī lokadhāriṇī .
uddhṛtāsi varāheṇa kṛṣṇena śatabāhunā .. 38..
- The earth is the giver of happiness like the milk cow, the sustainer of life and support for all living beings. (Represented as such the earth is addressed:)Thou wert raised up by Kṛṣṇa in His incarnation of the boar having hundred hands.
Taittiriya Samhita Kanda 7
This was in the beginning the waters, the ocean. In it Prajapati becoming the wind moved. He saw her, and becoming a boar he seized her. Her, becoming Viçvakarma, he wiped. She extended, she became the earth, and hence the earth is called the earth (lit. 'the extended'). In her Prajapati made effort. He produced the gods, Vasus, Rudras, and Adityas.
Taittiriya Samhita Kanda 6
Now a boar, stealer of the good, keeps the wealth of the Asuras which is to be won beyond the seven hills. Him smite, if thou art he who smites in the stronghold. He [Indra] plucked out a bunch of Darbha grass, pierced the seven hills, and smote him. He said, 'Thou art called he who brings from the stronghold; bring him.' So the sacrifice bore off the sacrifice for them; in that they won the wealth of the Asuras which was to be won (védyam), that alone is the reason why the Vedi is so called. The Asuras indeed at first owned the earth, the gods had so much as one seated can espy.