Here are the relevant lines from stanza 2:
> And from this chasm, with ceaseless turmoil seething,
> As if this earth in fast thick pants were breathing,
> A mighty fountain momently was forced:
> Amid whose swift half-intermitted burst
> Huge fragments vaulted like rebounding hail,
> Or chaffy grain beneath the threshing flail:
> And from the fount as it would overflow,
> With its gushing and its jovial troop,
> The waters rise and fall -- but never go,
> And from the cavern on the ocean pour,
> Through rocky valleys into the depths below.
It's important to note that the speaker doesn't explicitly say what is found inside the cavern, but rather describes its characteristics and the water pouring out of it. The description of the "sunless sea" and "shadows of the dome of pleasure" suggests a mysterious and perhaps even otherworldly place.