What is the sunless sea in kubla khan?

In Samuel Taylor Coleridge's poem "Kubla Khan," the "sunless sea" symbolizes a mysterious and otherworldly realm, contrasting with the vibrant and lush imagery surrounding the pleasure dome. It evokes a sense of depth, darkness, and the unknown, suggesting both the beauty and the potential danger of nature. This sea reflects the poem's themes of imagination, creativity, and the sublime, embodying the tension between light and shadow in the human experience.