The 'sacred river' Alph ran 'five miles meandering with a mazy motion' by Kubla's 'pleasure dome', and then dissolved itself down into the 'sunless sea'. The alliterative 'sunless sea' conjures up a visual image of vast, tumultous darkness. But is this an image of nocturnal darkness when there is no sun in the sky? Or does it refer to a more elemental darkness when/where there is no sun at all? Coleridge tells that the river Alph went underground through 'caverns measureless to man' down to the 'sunless sea', which means that the dark sea is a subterranean sea. Since we are not aware of any such sea, the image is one of dreadful mystery & confusion.
If the 'sacred river' Alph symbolises life and the same dissolves into the roaring depths of the 'sunless sea', the mysteriously dreadful sea must be symbolic of death. This 'sunless sea' of stanza 1 is called the 'lifeless ocean' in stanza 2. Since the sun is the source of light & life on earth, sunlessness means lifelessness; and the meandering river of life mingles into the mysterious darkness of death.
Even if the poem is the outcome of an opium dream, it has to be studied as poetry by the critics . In seeking the significance of the poem there is often a tendency to discover some kind of allegory or symbolic meaning.The river Alph may stand for life. For, it is described as sacred and sinks into the sunless sea of death. The river representing life , he writes "seethes from the earth itself, flows slowly and with a mazy motion, expressive perhaps of life's twists and turns , through a walled garden and at last enters the caverns measureless to man before sinking to a lifeless ocean. The sunny dome, standing in contrast to the sunless sea, is the poetic imagination. It is a proud manifestation of life: it is both a work of superb art in itself and a symbol for all artistic achievements.The main reason for thinking that the dome is the symbol for the creative imagination is the poet's declaration that he would build the dome in air.