You must be thinking of another poem. Coleridge's "Kubla Khan" has nothing to do with the underworld or venturing to the dark world beneath.
As has already been mentioned in answers to a couple of other questions, Coleridge was inspired to write this poem by a dream. He had taken some medicine that made him drowsy, and he was reading a book about the Kubla Khan just as he was drifting off to sleep. When he woke up, he wrote down everthing he could remember.
This poem is very complex, and it is difficult to say what its theme might be, other than a poetic description of Kubla Khan's kingdom. The eNotes study guide states that
most assessments of “Kubla Khan” remain unable to answer with any degree of certainty the question of the poem's ultimate meaning. In part due to its status as a verse fragment and the continued controversy surrounding its origins, “Kubla Khan” has tended to discourage final interpretation. Nevertheless, most critics acknowledge that the juxtaposed images, motifs, and ideas explored in the poem are strongly representative of Romantic poetry.
The sheer magic of Coleridge's poem Kubla Khan eludes every attempt to arrive at an explanation, clear and conclusive.
In Stanza 1, Coleridge presents a 'sacred river' called Alph, a mystical, extra-geographical river which runs overground for a few miles before dissolving itself into a sub-terranean 'sunless sea' by jumping through 'caverns measureless to man'. This is an underworld, eerie & invisible, and full of commotion.
In Stanza 2, we are told of the birth of the river from a 'mighty fountain' at the bottom of a 'deep romantic chasm'.The upjetting water of the fountain thrusts itself upward in 'half-intermitted bursts' producing yet another turmoil, this time a commotion of a perpetual moment of the birth of life. Pieces of rocks being thrown upwards assume a dancing pattern to give us a combined impression of violence and harmony.
Thus the two underworlds respectively symbolise the dark, invisible world of death & the deeply embedded world of life being perpetually born.It is the river of life--'five miles meandering with a mazy motion'--which is being continually born at one end, and perpetually dying at the other.