Why is Hindu god Vishnu is shown with a mutiheaded snake above his head?

Why is Hindu god Vishnu is shown with a mutiheaded snake above his head?

1) Adishesha:

In Hindu (Vedic) tradition, Shesha (शेष) is the king of all nagas, one of the primal beings of creation, and according to the Bhagavata Purana, an avatar of the Supreme God known as Sankarshan.

In the Puranas, Shesha is said to hold all the planets of the Universe on his hoods and to constantly sing the glories of Vishnu from all his mouths.

"Adishesha", which means First snake.

2) Ananta-Shesha:

Shesha is generally depicted with a massive form that floats coiled in space, or on the universal ocean, to form the bed on which Vishnu lies. Sometimes he is shown as five-headed or seven-headed, but more commonly as a many hundred-headed serpent, sometimes with each head wearing an ornate crown.

He is closely associated with Vishnu. His name means "that which remains", from the Sanskrit root sis, because when the world is destroyed at the end of the kalpa, Shesha remains as he is.

So he is refered as "Ananta-Shesha" which means "Endless Shesha".

Lord Vishnu is seen resting on the coils of the serpent Shesha, also called Ananta. Sheshanaga is the expansion of Lord Balarama, Lord Krishna’s brother, and serves the Lord in this way as the Lord’s support and paraphernalia. Shesha has a thousand heads swinging to and fro over the form of Lord Vishnu, creating a shelter and couch for the Lord.

Ananta is endlessly singing the praises and glories of the Lord from His thousand hoods without ever reaching the end. His hoods are also supporting the many planetary systems in the cosmic creation that are orbiting throughout the universe above His heads. Ananta also means endless in terms of the endlessness of cosmic time. This is also represented by His thousand hoods as divisions of time. The material worlds are created within the element of time, and are thus sustained by time. During the process of the universal annihilation, time ceases to exist, in which case the material planets are also forced into obliteration.

Lord Shesha is often seen floating on the causal waters of the Garbhodaka Ocean, which exists on the bottom of the universe. Lord Vishnu is thus resting on Sheshanaga as They float on the ocean. At other times, They are viewed floating on the Kshiramudra, or an ocean of white milk. This represents the Prakriti or the ingredients of the unmanifest material nature in its purest form.

3) Ananda Sesha as incarnation of Godhead:

(i) Srimad Bhagavatam 10.1.24:

"The foremost manifestation of Krishna is Sankarshana, who is known as Ananta. He is the origin of all incarnations within this material world. Previous to the appearance of Lord Krishna, this original Sankarshana will appear as Baladeva, just to please the Supreme Lord Krishna in His transcendental pastimes."

(ii) Srimad Bhagavatam 5.25.1:

"My dear King, approximately 240,000 miles beneath the planet Patala lives another incarnation of the Supreme Personality of Godhead. He is the expansion of Lord Vishnu known as Lord Ananta or Lord Sankarshana. He is always in the transcendental position, but because He is worshiped by Lord Siva, the deity of tamo-guna or darkness, He is sometimes called Tamasi.

Lord Ananta is the predominating Deity of the material mode of ignorance as well as the false ego of all conditioned souls. When a conditioned living being thinks, 'I am the enjoyer, and this world is meant to be enjoyed by me,' this conception of life is dictated to him by Sankarshana. Thus the mundane conditioned soul thinks himself the Supreme Lord."

(iii) Sri Chaitanya Caritamrita Adi-lila 5.120:

"That Ananta Sesha is the devotee incarnation of Godhead. He knows nothing but service to Lord Krishna."

(iv) Srila Jiva Gosvami, in his Krishna-sandarbha:

"Sri Anantadeva has thousands of faces and is fully independent. Always ready to serve the Supreme Personality of Godhead, He waits upon Him constantly. Sankarshana is the first expansion of Vasudeva, and because He appears by His own will, He is called svarat, fully independent. He is therefore infinite and transcendental to all limits of time and space. He Himself appears as the thousand-headed Sesha."

(v) A. C. Bhaktivedanta Swami Prabhupada:

"Sankarshana of the quadruple form descends with Lord Rama as Lakshmana. When Lord Rama disappears, Sesha again separates Himself from the personality of Lakshmana. Sesha then returns to His own abode in the Patala regions, and Lakshmana returns to His abode in Vaikuntha."

(vi) Bhagavad-Gita, Chapter 10, Verse 29:

In the Bhagavad-Gita, when in the middle of the battlefied Kurukshetra, Krishna explaining his omnipresence, says: "Of Nagas, I am Ananta" indicating the importance of Ananta Shesha.

4) Avataras of Adishesha:

Balarama, Lakshmana, and Nityananda Prabhu, are considered avataras of Shesha.

Patañjali is also considered an emanation or incarnation of Shesha and is iconographically depicted in naga form with naga canopy.

5) More on Adishesha:

In a story from the Puranas, Shesha loosens Mount Mandara, to enable it to be used in the churning of the ocean by the devas and asuras.

According to the Mahabharata (Adi Parva), his father was Kashyapa and his mother Kadru.

The city of Thiruvananthapuram is named after him as the "City of Lord Ananta".