Tolkien describes two entrances to the Lonely Mountain: the main gate and the secret door. Both are described in similar fashion.
At the main gate: There was a “dark cavernous opening” and “out of it too there came a steam and a dark smoke. Nothing moved in the waste, save the vapour and the water” (217).
And after the secret door was opened: “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” (223).
Whatever or whoever is under the Lonely Mountain, Tolkien wants us to know that darkness is what awaits the party—in more ways than one.