What are they entering? What is the symbolic significance of the hole? What is the symbolic significance of the vapourous darkness? How does this image relate to the heroic journey?

Bilbo and the dwarves have scaled lonely mountain, the secret door has been opened, and this is what awaits them: It seemed as if darkness flowed out like a vapour from the hole in the mountain-side, and deep darkness in which nothing could be seen lay before their eyes, a yawning mouth leading in and down.