Arjuna had broken a promise and had to go to exile for twelve years. While he was in the forest, a beautiful woman saw him and fell in love with him. She was actually a snake, called Ulupi . She was the daughter of the king of snakes, Kauravya, and lived in the snake kingdom in patal lok (netherworld).
Next day, when Arjuna went to take a bath in the river, Ulupi took form of snake and caught hold of his leg and dragged him patal lok. There, She took him to her palace and asked Arjuna to marry her. Arjuna agreed. They got married and had a son names Iravan. Then Arjuna wanted to go back to earth and Ulupi took him there. She also gave him a boon that no creature of water could defeat him.