In the Causal Ocean there are innumerable Brahmās rising and disappearing like bubbles in the Atlantic. Brahmā and his creation are all part of the material universe, and therefore they are in constant flux.
(Bhagavad-Gita-----8:17-----purport).

At the creation, the material energy is let loose as the mahat-tattva, into which the Lord as His first puruṣa incarnation, Mahā-Viṣṇu, enters. He lies within the Causal Ocean and breathes out innumerable universes, and into each universe the Lord again enters as Garbhodakaśāyī Viṣṇu.
(Bhagavad-Gita-----9:8-----purport).

Nothing can take place without a cause. The Kāraṇa Ocean is therefore called the Causal Ocean. Kāraṇa means "causal." We should not foolishly accept the atheistic theory of creation.
(Srimad Bhagavatam-----1:3:2-----purport).

In the Śruti-mantra it is said that only Viṣṇu, the Supreme Lord, existed before the creation, and there was no Brahmā, Śiva or other demigods. Viṣṇu means the Mahā-Viṣṇu, who is lying on the Causal Ocean. By His breathing only all the universes are generated in seeds and gradually develop into gigantic forms with innumerable planets within each and every universe.
(Srimad Bhagavatam-----1:10:21-----purport).

Beyond the material sky, further than we can see with our eyes and beyond the sevenfold coverings of the universe, there is the Causal Ocean in which all the universes are floating like footballs, and beyond the Causal Ocean there is an unlimited span of spiritual sky generally known as the effulgence of Brahman. Within this effulgence there are innumerable spiritual planets, and they are known as the Vaikuṇṭha planets.
(Srimad Bhagavatam-----1:14:35-36-----purport).

The effulgent luminary planets like the sun and the moon are situated almost in the midplace of the universe, and as such they are to be known as the chest of the original gigantic form of the Lord. And above the luminary planets, called also the heavenly places of the universal directorate demigods, are the Mahar, Janas and Tapas planetary systems, and, above all, the Satyaloka planetary system, where the chief directors of the modes of material nature reside, namely Viṣṇu, Brahmā and Śiva. This Viṣṇu is known as the Kṣīrodakaśāyī Viṣṇu, and He acts as the Supersoul in every living being. There are innumerable universes floating on the Causal Ocean, and in each of them the representation of the virāṭ form of the Lord is there along with innumerable suns, moons, heavenly demigods, Brahmās, Viṣṇus and Śivas, all of them situated in one part of the inconceivable potency of Lord Kṛṣṇa, as stated in the Bhagavad-gītā (10.42).
(Srimad Bhagavatam-----2:1:28-----purport).

In a corner of the spiritual sky of brahmajyoti a spiritual cloud sometimes appears, and the covered portion is called the mahat-tattva. 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. These universes are floating, and they are scattered all over the Causal Ocean. They stay only during the breathing period of Mahā-Viṣṇu. In each and every universal globe, the same Mahā-Viṣṇu enters again as Garbhodakaśāyī Viṣṇu and lies there on the serpentlike Śeṣa incarnation.
(Srimad Bhagavatam-----2:5:33-----purport).

The original Personality of Godhead (ādyaḥ), Govinda, expands Himself as the Mahā-Viṣṇu incarnation and rests in the Causal Ocean, which He Himself creates. The Brahma-saṁhitā (5.47) confirms this as follows:
yaḥ kāraṇārṇava-jale bhajati sma yoga-
nidrām ananta-jagad-aṇḍa-saroma-kūpaḥ
ādhāra-śaktim avalambya parāṁ sva-mūrtiṁ
govindam ādi-puruṣaṁ tam ahaṁ bhajāmi
Lord Brahmājī says in his Brahma-saṁhitā, "I worship the primeval Lord Govinda, who lies down in the Causal Ocean in His plenary portion as Mahā-Viṣṇu, with all the universes generating from the pores of hair on His transcendental body, and who accepts the mystic slumber of eternity."
So this Mahā-Viṣṇu is the first incarnation in the creation, and from Him all the universes are generated and all material manifestations are produced, one after another. The Causal Oceanis created by the Lord as the mahat-tattva, as a cloud in the spiritual sky, and is only a part of His different manifestations. The spiritual sky is an expansion of His personal rays, and He is the mahat-tattva cloud also.
(Srimad Bhagavatam-----2:6:39-----purport).

In the Brahma-saṁhitā (5.47-48) it is said that all the material manifestations with innumerable universes appear and disappear with the breathing of Mahā-Viṣṇu lying in yoga-nidrā, or mystic sleep.
yaḥ kāraṇārṇava-jale bhajati sma yoga-
nidrām ananta-jagad-aṇḍa-saroma-kūpaḥ
ādhāra-śaktim avalambya parāṁ sva-mūrtiṁ
govindam ādi-puruṣaṁ tam ahaṁ bhajāmi
yasyaika-niśvasita-kālam athāvalambya
jīvanti loma-vilajā jagad-aṇḍa-nāthāḥ
viṣṇur mahān sa iha yasya kalā-viśeṣo
govindam ādi-puruṣaṁ tam ahaṁ bhajāmi
"Govinda, the ultimate and Supreme Personality of Godhead (Lord Kṛṣṇa), lies sleeping unlimitedly on the Causal Ocean in order to create unlimited numbers of universes during that sleep. He lies on the water by His own internal potency, and I worship that original Supreme Godhead.
"Due to His breathing, innumerable universes come into existence, and when He withdraws His breath there occurs the dissolution of all the lords of the universes. That plenary portion of the Supreme Lord is called Mahā-Viṣṇu, and He is a part of the part of Lord Kṛṣṇa. I worship Govinda, the original Lord."
(Srimad Bhagavatam-----3:7:22-----purport).

Lord Mahā-Viṣṇu, who is resting in yoga-nidrā on the Causal Ocean and creating innumerable universes by His breathing process, only temporarily appears in the mahat-tattva for the temporary manifestation of the material worlds. He is a plenary portion of Lord Śrī Kṛṣṇa, and thus although He is nondifferent from Lord Kṛṣṇa, His formal appearance in the material world as an incarnation is temporary.
(Srimad Bhagavatam-----3:11:42-----purport).

The Lord can assume any form He likes, and in all circumstances He is the cause of all causes. Since His form is transcendental, He is always the Supreme Personality of Godhead, as He is in the Causal Ocean in the form of Mahā-Viṣṇu. Innumerable universes generate from the holes of His bodily hairs, and thus His transcendental body is the Vedas personified. He is the enjoyer of all sacrifices, and He is the unconquerable Supreme Personality of Godhead.
(Srimad Bhagavatam-----3:13:34-----purport).

Brahmā has confirmed that Lord Kṛṣṇa is the Supreme, the source of the three manifestations known as Kṣīrodakaśāyī Viṣṇu, Garbhodakaśāyī Viṣṇu and Kāraṇodakaśāyī Viṣṇu (Mahā-Viṣṇu). For His pastimes, Lord Kṛṣṇa has four original manifestations—namely Vāsudeva, Saṅkarṣaṇa, Pradyumna and Aniruddha. The first puruṣa-avatāra, Mahā-Viṣṇu in the Causal Ocean, who is the creator of the aggregate material energy, is an expansion of Saṅkarṣaṇa; the second puruṣa, Garbhodakaśāyī Viṣṇu, is an expansion of Pradyumna; and the third puruṣa, Kṣīrodakaśāyī Viṣṇu, is an expansion of Aniruddha. All these are within the category of manifestations of Nārāyaṇa, who is a manifestation of Śrī Kṛṣṇa.
(Sri Caitanya Caritamrta-----1:2:56-----purport).

The first descent of the Supreme Personality of Godhead from the expansion of Saṅkarṣaṇa is the puruṣa incarnation, Mahā-Viṣṇu. It is confirmed in Śrīmad-Bhāgavatam (1.3.1) that when the Supreme Personality of Godhead descends as the first puruṣa incarnation of the material creation, He immediately manifests sixteen elementary energies. Known as the Mahā-Viṣṇu, He lies within the Causal Ocean, and it is He who is the original incarnation in the material world. He is the Lord of time, nature, cause and effect, mind, ego, the five elements, the three modes of nature, the senses and the universal form. Although He is master of all objects movable and immovable in the material world, He is totally independent.
(Teachings of Lord Caitanya).

Kṛṣṇa is known as Yogeśvara. He has all the opulences or facilities of mystic powers, such as aṇimā-siddhi, the power to become smaller than the smallest. It is stated in the Brahma-saṁhitā that Kṛṣṇa has entered even within the atom (aṇḍāntara-stha-paramāṇu-cayāntara-stham (Bs. 5.35)). Similarly, Kṛṣṇa, as Garbhodakaśāyī Viṣṇu, is within the gigantic universe, and He is lying in the Causal Ocean as Mahā-Viṣṇu, in a body so gigantic that when He exhales, millions and trillions of universes emanate from His body. This is called mahimā-siddhi.
(Krsna Book).