Nagas
Of the celestial Naga snakes I am Ananta.
Bhagavad Gita, Chapter 10, verse 29 (Krishna
says)
The word "nag" is used even today in most Indian languages as a term for a snake, especially cobra.
Naga is the Sanskrit and Pali word for a deity or class of beings found in Hinduism and Buddhism, who dwell in underground premises of our earth. There are also legends about prehistoric Nagas in the folklore of present tribal Hindus of Southern India (Adivasis) and the aboriginals of Australia. In this legend, the Nagas inhabited a big continent that existed somewhere in the Pacific Ocean. It sank and the remnants formed the Indonesian archipelago and Australia. These Nagas are said to have developed a subterranean civilization technologically more advanced than ours and they are thought to possess superhuman powers.
Nagas are so called "snake people" and
they are mentioned in many Hindu texts. Arjuna married a Naga princes
Ulupi (Bhagavadgita) and there is also a Tamil text Kanzul Karamat,
which describes one Muslim saint as he was captured and transported
in Sri Lanka's Kataragama to a subterranean palace where he received
a mysterious robe. 
There is a Naga Purana or Nilamata Purana, which has its origin in Kashmir.
The chief of Nagas is Varuna, a Vedic god. A female Naga is called Nagini.
There is also an expansion of Vishnu called Shesha. Vishnu once assumed a form of a gigantic divine snake with thousands of heads.
An important underground-kingdom related story is associated with Vamana, or "dwarf incarnation" of Lord Vishnu, which occurred in Treta Yuga (the second age), long before the events described in Ramayana, and in strong association with the king Mahabali (some call him Maveli or Bali). Mahabali was a king of the Indian region presently known as Kerala (southern Indian state bordering with Tamil Nadu), who stands behind the most popular legend - the Onam legend, which is a festival in celebration of the King Mahabali, who returns to the state of Kerala from underground every year. Read how Lord Vishnu tested this king.
The story of Khodiyar Maa
goes back to around 700 AD and
begins with a childless man - Mamaniya Gadhvi, who had a superb
relationship with the then ruler - Maharaj Shilbhadra. The
ruler's ministers envied this exceptional relationship and prepared
a way to get rid of Mamaniya Gadhvi. They were not very successful
to persuade the ruler, but they succeeded in persuading the ruler's
wife (queen). One day the doorkeepers did not let him go into the
palace. Mamaniya asked for the reason. He was told that a childless
man is not worth of the king's presence. 
Mamaniya
returned home and wanted to ask Lord
Shiva for help. When Lord Shiva
did not appear, he decided to give his life away as a final sacrifice.
Just when he was about to make it, Lord Shiva
appeared and took him to the
Snake Kingdom - Naglok
(or Naga Loka) to see the King of Snakes
- Nagdev. After hearing his story full of humiliation,
the Nagdev's daughters decided to help too.
The picture of Khodiyar Maa is copyrighted and taken from http://www.flickr.com/ (search for the key word "Khodiyar Maa" to see more).
When Mamaniya came home, with his wife he
prepared, as advised by the Nagdev's daughters, eight
cradles in expectation of a great event. One day eight
snakes crawled into the house and Mamaniya had suddenly
seven daughters and one
son. One of the daughters was Khodiyar Maa. After
showing many miraculous powers, people consider her to be goddess
and she has temples and shrines too. Her vehicle is crocodile
and she has many other names such as Khodal, Trishuldhari, Maavdi...
