Once there was a Maharshi Mrikandu in the lineage of Bhrigu. He lived in a forest. There he got a son. When he became 5-year old, a Gyaanee saw him and thought about him. Mrikandu asked him about his son's total age. He told him that his son was left only with 6 months. Hearing this his father performed his Yagyopaveet Sanskaar and asked him to greet every Muni he came across with. He did that happily.

Gradually 5 months and 25 days passed. Once Saptarshi were passing by that path. The child greeted them too. They blessed him - "Aayushmaan Bhava, Saumya." (O Beautiful, have a long life.) After they blessed him, they thought about his age, and they found that he had onlyn5 days left. They got very sad and went to Brahmaa Jee. They took the child also with them. When the child saw Brahmaa Jee, he greeted him also. Brahmaa Jee also blessed him - "Aayushmaan Bhava." Rishi got very happy to hear this blessing for him.

Brahmaa Jee asked Rishi - "Why have you come here and who is this child?" Rishi said - "He is the son of Rishi Mrikandu. His age is getting over. By nature he greets everybody. One day we were passing by that side, that he greeted us and we blessed him with long life, now you have also blessed him with long life. We all should not be called liar." Brahmaa Jee said - "This boy will be like me in age and will survive several Kalp surrounded by great Munis." After this Rishi sent him to Prithvi Lok.

The boy said to his father - "Some Rishi took me to Brahm Lok. There Brahmaa Jee blessed me for long life, so you should not worry about my life. Now I will go to Pushkar Teerth." His father got very happy to hear this, he said - "Today I am blessed that you had the Darshan of Brahmaa Jee. Who has his Darshan, he doesn't have to fear of old age and death. What I could not do, you did it very easily."
[Padm Puraan, 1/17]

Another Version
There is another version of the same story. Mrikandu Rishi and his wife were childless. So he did intense Tap to get a child. Bhagavaan Shiv appeared before him and asked him to ask for a Var. He said - "You know Prabhu, I am childless, please give me a son." Shiv Jee said - "You have two alternatives - one is that you can have an ordinary child with long life, or two, you can have an intelligent child with only 16 years of age." Mrikandu Rishi thought for a while and then opted for the intelligent child. After some time he had a son. The couple got very happy to see that child. He started to bring him very lovingly. He completed his all Sanskaars methodically.

The child started growing. Rishi taught him whatever he could teach. Gradually his death day drew nearer and nearer. When only one month left of his life, he became very sad. Seeing his father sad, one day the child asked him - "Father, Why are you so sad?" The father told him the whole story. The child was a great devotee of Shiv, he said - "Father, You do not worry, Shiv is very kind to everybody. He has saved many people from death. I will worship Shiv from today, I am sure, he will save me also from death." So he went to a Shiv temple and started worshipping Shiv with great devotion. The day he was supposed to die, he started doing Jaap of Mrityunjaya Mantra.

He was doing it that Yam Doot came to take him and threw the death noose in his neck. He said - "Wait, let me finish my worship, then you may take me." and continued to do his Jaap. But the time was passing so the Doot again tried to throw noose in his neck, the child again said - "I told you to wait. Let me finish my worship." At this Yam Doot got angry and they again tried to throw death noose in his neck. But this time the noose fell around Shiv Ling the child was worshipping. As they pulled the noose, the Ling broke out and Shiv came out of it. Seeing Shiv, Yam Doot ran away. Shiv was very pleased with the child, he blessed him with immortality and disappeared.

Thus the child who was supposed to die at the age of 16, became immortal. Mrikandu Muni got very happy to find his son alive and well.

Maarkandeya Saw the Great Deluge
One time Maarkandeya Muni did great Tapasyaa. Pleased by his Tap, Vishnu appeared and asked him to ask for any Var. Muni told that he wanted to see Pralaya. "So be it, On the 8th day from today you will see the Pralaya." and he disappeared. Muni forgot this and started doing his daily work. On the 8th day, the black clouds started appearing and in a little while rain started pouring heavily. Muni thought, "this is not the time of rain, where it has come from?" Seas forgot their limits and started crossing them. The whole Prithvi started sinking in water. Muni's worshipping materials flowed in the water. Even his hut started being covered with water. Muni did not know swimming, he started drowning in water but he did not drown in water. Sometimes he came up and sometimes he went under the water. After a while the whole Prithvi sank in the water and Muni floated on the water. There was no land on which Muni could set his foot.

Muni did not know how many days or months or even years passed like this. One time he saw a Vat tree at some distance. He thought "where this tree has come from in this water?" He proceeded a little bit towards it, so that he could take some rest on that tree, as he was tired floating on the water, that he saw a branch bent towards water and its leaves had formed a large cup, and in that cup was a newborn baby lying and sucking on his big right toe.

Muni was so surprised to see this, he started going towards that tree, but as he wanted to reach nearer to that tree, it went far from him. Once he happens to reach very near to it and he wanted to touch the child that His silent breath draws Maarkandeya inside Himself. Time was circulating inside Him in fluid form. He saw many things in His stomach, he doesn't remember. Later he came out from His stomach through His exhale. He was again standing beside His cradle. Maarkandeya opens his mouth in the ecstasy of this vision.... that ZAP..... and the whole scene of Deluge had disappeared.

And Muni found himself standing in his hut.