Srimad-Bhagavatam
SB Canto 1
SB 1.3.2, Purport: The Mahā-Viṣṇu lies down in some part of the spiritual sky by His own free will. Thus He lies on the ocean of kāraṇa, from where He glances over His material nature, and the mahat-tattva is at once created. Thus electrified by the power of the Lord, the material nature at once creates innumerable universes, just as in due course a tree decorates itself with innumerable grown fruits. The seed of the tree is sown by the cultivator, and the tree or creeper in due course becomes manifested with so many fruits. Nothing can take place without a cause. The Kāraṇa Ocean is therefore called the Causal Ocean. Kāraṇa means "causal."
SB Canto 2
SB 2.5.33, Purport: The Lord then, by His plenary portion as Mahā-Viṣṇu, lies down within the water of the mahat-tattva, and the water is called the Causal Ocean (Kāraṇa-jala). While Mahā-Viṣṇu sleeps within the Causal Ocean, innumerable universes are generated along with His breathing.
SB Canto 3
SB 3.20.17, Purport: At a certain time, the Personality of Godhead, Kāraṇodakaśāyī Viṣṇu, lies in the Kāraṇa Ocean and produces many thousands of universes from His breathing; then He enters again into each and every universe as Garbhodakaśāyī Viṣṇu and fills up half of each universe with His own perspiration. The other half of the universe remains vacant, and that vacant region is called outer space.
Sri Caitanya-caritamrta
CC Adi-lila
CC Adi 1.7, Translation: May Śrī Nityānanda Rāma be the object of my constant remembrance. Saṅkarṣaṇa, Śeṣa Nāga and the Viṣṇus who lie on the Kāraṇa Ocean, Garbha Ocean and ocean of milk are His plenary portions and the portions of His plenary portions.
CC Adi 1.9, Translation: I offer my full obeisances unto the feet of Śrī Nityānanda Rāma, whose partial representation called Kāraṇodakaśāyī Viṣṇu, lying on the Kāraṇa Ocean, is the original puruṣa, the master of the illusory energy, and the shelter of all the universes.
CC Adi 2.47, Translation: Kṛṣṇa said, “Brahmā, I cannot understand what you are saying. Lord Nārāyaṇa is He who sits in the hearts of all living beings and lies down in the waters of the Kāraṇa Ocean.”
CC Adi 2.52, Purport: The three puruṣas—Kāraṇodakaśāyī Viṣṇu, Garbhodakaśāyī Viṣṇu and Kṣīrodakaśāyī Viṣṇu—all have a relationship with the material energy, called māyā, because through māyā They create the material cosmos. These three puruṣas, who lie on the Kāraṇa, Garbha and Kṣīra oceans respectively, are the Supersoul of everything that be: Kāraṇodakaśāyī Viṣṇu is the Supersoul of the collective universes, Garbhodakaśāyī Viṣṇu is the Supersoul of the collective living beings, and Kṣīrodakaśāyī Viṣṇu is the Supersoul of all individual living entities.
CC Adi 2.91-92, Purport: (8) Nirodha: the winding up of all energies employed in creation. Such potencies are emanations from the Personality of Godhead who eternally lies in the Kāraṇa Ocean. The cosmic creations, manifested with His breath, are again dissolved in due course.
CC Adi 5, Purport: Outside of the Vaikuṇṭha planets is the impersonal manifestation of Śrī Kṛṣṇa, which is known as Brahmaloka. On the other side of Brahmaloka is the spiritual kāraṇa-samudra, or Causal Ocean.
CC Adi 5.7, Translation: May Śrī Nityānanda Rāma be the object of my constant remembrance. Saṅkarṣaṇa, Śeṣa Nāga and the Viṣṇus who lie on the Kāraṇa Ocean, Garbha Ocean and ocean of milk are His plenary portions and the portions of His plenary portions.
CC Adi 5.50, Translation: I offer my full obeisances unto the feet of Śrī Nityānanda Rāma, whose partial representation called Kāraṇodakaśāyī Viṣṇu, lying on the Kāraṇa Ocean, is the original puruṣa, the master of the illusory energy, and the shelter of all the universes.
CC Adi 5.51, Translation: Outside the Vaikuṇṭha planets is the impersonal Brahman effulgence, and beyond that effulgence is the Kāraṇa Ocean, or Causal Ocean.
CC Adi 5.54, Translation: The water of the Kāraṇa Ocean, which is the original cause, is therefore spiritual. The sacred Ganges, which is but a drop of it, purifies the fallen souls.
CC Adi 5.57, Translation: Māyā-śakti resides outside the Kāraṇa Ocean. Māyā cannot touch its waters.
Other Books by Srila Prabhupada
Krsna, The Supreme Personality of Godhead
Krsna Book 89: After this, Lord Kṛṣṇa and Arjuna entered a vast spiritual water. This spiritual water is called the Kāraṇa Ocean, which means that this ocean is the origin of the creation of the material world; this place is also known as Virajā, because it is free from the influence of the three qualities of the material world. In the Mṛtyuñjaya-tantra, a Vedic scripture, there is a vivid description of this Kāraṇa Ocean, or Virajā. It is stated there that the highest planetary system within the material world is Satyaloka, or Brahmaloka, beyond which are Rudraloka and Mahā-Viṣṇuloka. Regarding this Mahā-Viṣṇuloka, it is stated in the Brahma-saṁhitā, yaḥ kāraṇārṇava-jale bhajati sma yoga-nidrām ananta-jagad-aṇḍa-sa-roma-kūpaḥ: [Bs. 5.47] “Lord Mahā-Viṣṇu is lying in the Kāraṇa Ocean. When He exhales, innumerable universes come into existence, and when He inhales, innumerable universes enter within Him.” In this way, the material creation is generated and again withdrawn. When Lord Kṛṣṇa and Arjuna entered the water, it appeared that there was a strong hurricane of transcendental effulgence blowing, and the water of the Kāraṇa Ocean was greatly agitated. By the grace of Lord Kṛṣṇa, Arjuna had the unique experience of being able to see the very beautiful Kāraṇa Ocean.
Lectures
Bhagavad-gita As It Is Lectures
Lecture on BG 4.10 -- Bombay, March 30, 1974: And innumerable universes are coming out from the hair holes of the body of Mahā-Viṣṇu. Niśvasita-kālam athāvalambya. His breathing. He is, within the Kāraṇa Ocean, Causal Ocean, He is sleeping. Yaḥ kāraṇārṇava-jale bhajati sma yoga-nidrām [Bs. 5.47]. Yoga-nidrā. And while He's sleeping in His yoga-nidrā, innumerable universes are coming out through His breathing and from the air holes.
Lecture on BG 7.4 -- Bombay, February 19, 1974: The material energy is coming through the breathing of Mahā-Viṣṇu, who is lying down in the Kāraṇa, Causal Ocean. So, from the breathing there is sound. From the ether, the sound comes—this is scientific—and from the sound, the air comes, the fire comes, then water comes, then the earth comes. The earth is the last, gross element in the material science. Therefore Kṛṣṇa says this earth, bhūmi... Bhūmi, earth, has come out of water, āpaḥ. And wherefrom āpaḥ, water, has come? Analaḥ, fire. Wherefrom fire has come? From air. Where the air has come? Ether, sky. Wherefrom the ether has come? False ego. Where from it has come? Mind. Wherefrom it has come? Intelligence. And wherefrom it has come? From the soul.
Srimad-Bhagavatam Lectures
Lecture on SB 3.26.25 -- Bombay, January 2, 1975: That Saṅkarṣaṇa, before this material creation, He lies down in the Kāraṇa Ocean, kāraṇa-toya-śāyī. Then... Yasyaika-niśvasita-kālam athāvalambya [Bs. 5.48]. He is lying in the Kāraṇa Ocean in the sleeping mood, and then, from His breathing, innumerable universes are coming out. That is the origin of material creation.
Sri Caitanya-caritamrta Lectures
Lecture on CC Adi-lila 1.7 -- Mayapur, March 31, 1975:
Nitāi: "May Śrī Nityānanda Rāma be the object of my constant remembrance. Saṅkarṣaṇa, Śeṣa Nāga and the Viṣṇus who lie on the Kāraṇa Ocean, Garbha Ocean and ocean of milk are His plenary portions and the portions of His plenary portions."
Prabhupāda:
saṅkarṣaṇaḥ kāraṇa-toya-śāyī
garbhoda-śāyī ca payobdhi-śāyī,
śeṣaś ca yasyāṁśa-kalāḥ sa nityā-
nandākhya-rāmaḥ śaraṇaṁ mamāstu
[Cc. Ādi 1.7]
So Kavirāja Gosvāmī, after offering his respectful obeisances to Śrī Caitanya Mahāprabhu and describing the purpose of His incarnation, He is now trying to describe Nityānanda.
Conversations and Morning Walks
1970 Conversations and Morning Walks
Room Conversation -- December 13, 1970, Indore: Prabhupāda: Yes. Kāraṇārṇava. Kāraṇārṇava. The Kāraṇa Ocean wherefrom the beginning of creation, material creation, that is nirvāṇa.
taken from here