Trilokapathagamini is one among the numerous names of Ganga.
Trilokapathagamini means flowing in three worlds. Ganga crossed over from
heaven to earth and then to the underworld or patala. Thus, she flowed in the
three worlds.
Ganga flowed in the heavens and King Sagara and his seven
generations performed intense austerities and tapas to bring Ganga down to
earth. King Bhagirath, great grandson of King Sagara, succeeded in attaining
the boon to bring Ganga down to earth.
As the torrential flow of Ganga could destroy the earth,
Shiva caught the river on his matted locks before releasing her to flow on
earth (you can read the story here).
Ganga then followed King Bhagirath from mountains to plains
to the sea. From there, Ganga River reached the nether regions or Patala where
she became the liberating waters for the sons of Sagara.
Related
Story of King Bhagirath bringing Ganga down to earth
Related
Story of King Bhagirath bringing Ganga down to earth