There is no direct reference to the underworld in "Kubla Khan" by Samuel Taylor Coleridge. However, the poem does reference "caverns measureless to man" (4), and this image is central to the poem. If we are to consider these caverns as an underworld, then we might say that they symbolize unconscious human brain power or the creative spirit that spurs on the imagination or artistic process.
It's important to understand that this poem is an enigma, and no scholar has been able to pin down an exact meaning for it. There are lots of ideas, of course, but no completely certain consensus (and that's one of the reasons this poem remains so compelling). However, many readers have noted that the poem seems to be exploring the capabilities of the imagination or of the artistic, creative process. As such, it's possible to say that the caverns in the poem (an underworld, perhaps) symbolize the subconscious that makes this imaginative, artistic process possible. It is impossible to say where this "underworld" goes (remember, the caverns are measureless to man), and it's also difficult to say what happens when one goes too far. However, as the poem includes some ominously violent imagery ("Ancestral voices prophesying war!" (30), for instance) it might be possible to surmise that venturing too far into one's imaginative subconscious has some frightening consequences.