Lord Shiva is often worshipped through the linga. Linga represent the male power of Shiva, his power and presence. They are oftern surrounded by the yoni, the female source of life. Together they symbolise male and female energy personified by Shiva and Shakti.
There are thirteen Jyothirlinga. All are described as being self formed icons of egg shape. Of all the Jyothirlingas, the thirteenth is considered to be very close to descriptions in the ancient Scripture, “Shiv-Puran”. The 13th Jyothirlinga, is called Mukti-Gupteshwar-Mahadev, one of the 1129 names for Lord Shiva. The other twelve lingas are called: Somnath, Mallikarjuna, Mahakaal, Mamaleshwar, Waidyanaath, Bhimshankar, Ramnath, Naagnath, Vishwanath, Trayambak, Kedarnath and Ghushmanatham.
The story of formation of the 13th Jyothirlinga, Mukti-Gupteshwar-Mahadev, is written in the Mahabharata. It is summarised as follows: Following a battle(jyood) the defeated Pandavas were banished into the forest for twelve years. During their period of exile memebers of the Pandavas arrived at large ashram located in Nepal. This ashram was central place for meditation.
According to the Scriptures, until this time the 13th Jyothirlinga or Shivlinga lay in the Ashram for 7000 years. While living in the Ashram, Apanda Arjun, a Pandavas warrior, went hunting. Confronted by a wild boar Arjuna fixed an arrow in his bow intending to kill it. At this moment another hunter claimed the boar as his game. Both hunters released theri arrows striking the boar at the same time. The two argued over the kill and fought for 21 days. As the Arjuna weakened he made a linga from mud and prayed only to find that his opponent was also Lord Shiva. Shiva had been testing the valour and skills of Arjuna. Shiva was pleased and said that he would be with him and also with whoever prays that Linga.