There is a lack of concrete evidence regarding birth and life history of Mahavatar Babaji Maharaj, the fountainhead of Kriya Yoga sadhana. Yogiraj  Lahiri Mahasaya has referred his Holiness as Babaji Maharaj who is also known as Mahamuni Babaji, Trayambak Baba, Babaji Nagraj and Shiva Baba. Maintaining a very high degree of secrecy Babaji Maharaj has been blessing from time to time many fortunate sadhak-sadhikas by giving them his divine darshan. Higher kriyabans during the practice of kriya yoga sadhana have felt the presence and essence of Babaji Maharaj. There is much hearsay about the invincibility and immortality of Babaji Maharaj with some sadhaks even claiming him to be the reincarnation of Lord Shiva. In the Kriya Yoga lineage Babaji Maharaj occupies the highest position being credited with transmitting kriya yoga techniques to revered Shyama Charan Lahiri Mahasay for its propagation amongst ordinary householders for the betterment of mankind. The said kriya yoga techniques were originally given to Arjuna by Bhagvan Sri Krishna and remained mainly confined to sannyasis as a means to self-realisation.

From Wikipedia: ” Mahavatar Babaji (IAST: Mahāvatāra Bābājī; literally; Great Avatar (Revered) Father or Elder or Wise One) is the name given to an Indian yogi by Yogiraj Lahiri Mahasaya, and several of his disciples, Sri Yukteswar Giri, Baba Nasib Singh Ji, Ram Gopal Muzumdar, Swami Kebalananda, and Swami Pranabananda Giri who reported meeting him between 1861 and 1935. Some of these meetings were described by Paramahansa Yogananda in his 1946 book Autobiography of a Yogi, including a report of Yogananda’s own meeting with the yogi.^[2] Another account was given by Sri Yukteswar Giri in his 1894 book The Holy Science.^[3] According to Sri M‘s autobiography (apprenticed to a Himalayan Master), Babaji was Lord Shiva. In the second to last chapter of his book, he mentions Babaji changing his form to that of Lord Shiva. All of these accounts, along with additional reported meetings, are described in various biographies.^[4][5][6] According to Yogananda’s autobiography, Babaji has resided for at least hundreds of years in the remote Himalayan regions of India, seen in person by only a small number of disciples and others.^[2][7]”