Kṛṣṇa is known as Yogeśvara, as stated in the last portion of the Bhagavad-gītā. Yogeśvaro hariḥ: all mystic powers are under His control. In our experience we can see many human beings who have yogic mystic power and who sometimes perform very wonderful acts, but Kṛṣṇa is understood to be the master of all mystic power. Therefore, when He saw that His horses were hesitant to proceed into the darkness, He immediately released His disc, known as the Sudarśana cakra, which illuminated the sky a thousand times brighter than sunlight. The darkness of the covering of the universe is also a creation of Kṛṣṇa's, and the Sudarśana cakra is Kṛṣṇa's constant companion. Thus He penetrated the darkness by keeping the Sudarśana cakra before Him. Śrīmad-Bhāgavatam states that the Sudarśana cakra penetrated the darkness just as an arrow released from the Śārṅga bow of Lord Rāmacandra penetrated the army of Rāvaṇa. Su means "very nice," and darśana means "observation"; by the grace of Lord Kṛṣṇa's disc, Sudarśana, everything can be seen very nicely, and nothing can remain in darkness. Thus Lord Kṛṣṇa and Arjuna crossed over the great region of darkness covering the material universes.
Arjuna then saw the effulgence of light known as the brahma-jyotir. The brahma-jyotir is situated outside the covering of the material universes, and because it cannot be seen with our present eyes, this brahma-jyotir is sometimes called avyakta. This spiritual effulgence is the ultimate destination of the impersonalists known as Vedāntists. The brahma-jyotir is also described as ananta-pāram, unlimited and unfathomed. When Lord Kṛṣṇa and Arjuna reached this region of the brahma-jyotir, Arjuna could not tolerate the glaring effulgence, and he closed his eyes. Lord Kṛṣṇa's and Arjuna's reaching the brahma-jyotir region is described in the Hari-vaṁśa. In that portion of the Vedic literature, Kṛṣṇa informs Arjuna, "My dear Arjuna, the glaring effulgence, the transcendental light you are seeing, is My bodily rays. O chief of the descendants of Bharata, this brahma-jyotir is I Myself." As the sun disc and the sunshine cannot be separated, Kṛṣṇa and His bodily rays, the brahma-jyotir, cannot be separated. Thus Kṛṣṇa claims that the brahma-jyotir is He Himself. This is clearly stated in the Hari-vaṁśa, when Kṛṣṇa says ahaṁ saḥ. The brahma-jyotir is a combination of the minute particles known as spiritual sparks, or the living entities, known as cit-kaṇa. The Vedic words so ’ham, or "I am the brahma-jyotir," can also be applied to the living entities, who can also claim to belong to the brahma-jyotir. In the Hari-vaṁśa, Kṛṣṇa further explains, "This brahma-jyotir is an expansion of My spiritual energy."
Kṛṣṇa told Arjuna, "The brahma-jyotir is beyond the region of My external energy, known as māyā-śakti." When one is situated within the material world, it is not possible to experience this Brahman effulgence. In other words, in the material world this effulgence is not manifested, whereas in the spiritual world it is manifested. That is the purport of the words vyakta-avyakta in the Hari-vaṁśa. In the Bhagavad-gītā it is said, avyakto ’vyaktāt sanātanaḥ: both these energies are eternally manifested.
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.
Accompanied by Kṛṣṇa, Arjuna saw a large palace within the water. There were many thousands of pillars and columns made of valuable jewels, and the glaring effulgence of those columns was so beautiful that Arjuna was charmed by it. Within that palace, Arjuna and Kṛṣṇa saw the gigantic form of Anantadeva, who is also known as Śeṣa. Lord Anantadeva, or Śeṣa Nāga, was in the form of a great serpent with thousands of hoods, each one decorated with valuable, effulgent jewels, beautifully dazzling. Each of Anantadeva's hoods had two eyes, which appeared very fearful. His body was as white as the mountaintop of Kailāsa, which is always covered with snow. His necks were bluish, as were His tongues. Thus Arjuna saw the Śeṣa Nāga form, and he also saw that on the very soft, white body of Śeṣa Nāga, Lord Mahā-Viṣṇu was lying very comfortably. He appeared all-pervading and very powerful, and Arjuna could understand that the Supreme Personality of Godhead in that form is known as Puruṣottama. He is known as Puruṣottama, the supreme or best Personality of Godhead, because from this form emanates within the material world another form of Viṣṇu, known as Garbhodakaśāyī Viṣṇu. The Mahā-Viṣṇu form of the Lord is also called Puruṣottama (Puruṣa-uttama) because He is beyond the material world. Tama means "darkness," and ut means "above, transcendental"; therefore, uttama means "above the darkest region of the material world." Arjuna saw that the bodily color of Puruṣottama, Mahā-Viṣṇu, was as dark as a new cloud in the rainy season. He was dressed in very nice yellow clothing, His face was beautifully smiling, and His eyes, which were like lotus petals, were very attractive. Lord Mahā-Viṣṇu's helmet was bedecked with valuable jewels, and His beautiful earrings enhanced the beauty of the curling hair on His head. Lord Mahā-Viṣṇu had eight arms, all very long, reaching to His knees. His neck was decorated with the Kaustubha jewel, and His chest was marked with the symbol of Śrīvatsa, which means "the resting place of the goddess of fortune." The Lord wore a garland of lotus flowers down to His knees. This long garland is known as a Vaijayantī garland.