Meaning of the last stanza of "Haunted Houses" by Longfellow?

Question

In the poem "Haunted Houses" by Henry Wadsworth Longfellow, the last two stanzas read:

And as the moon from some dark gate of cloud
Throws o'er the sea a floating bridge of light,
Across whose trembling planks our fancies crowd
Into the realm of mystery and night,—

So from the world of spirits there descends
A bridge of light, connecting it with this,
O'er whose unsteady floor, that sways and bends,
Wander our thoughts above the dark abyss.

What is the meaning of the last stanza, both literally and symbolically?

Answer 1

When the moon is low in the sky, it can cast a long reflection on bodies of water such as the sea. This can be seen in this picture, for example:

Longfellow poetically interprets this bright reflection as "a floating bridge of light", a bridge that connects the Earth to the heavens, or "world of spirits", above. The waves and ripples on the surface of the sea cause the reflection to jump and shimmer unpredictably. The second stanza thus describes the "bridge" as having an "unsteady floor, that sways and bends". A bridge typically spans a geographical feature like a valley or a gorge. In the case of this bridge, the final lines of the stanza tell that our thoughts wander along the bridge "above the dark abyss". This could literally mean the sea itself, or metaphorically, our earthly world from which our thoughts are ascending to the mysterious realm above.

Answer 2

The moon seen over the sea may cast a long reflection towards the viewer which, according to Longfellow, people imagine as a bridge leading to a mysterious region. He imagines a similar 'bridge of light' leading from us towards the spirit world, which would be precarious to walk across.

Answer 3

So from the world of spirits there descends A bridge of light, connecting it with this, O'er whose unsteady floor, that sways and bends, Wander our thoughts above the dark abyss.

From the world of spirits [in the sky], a bridge of light descends [comes down] and that bridge of light connects the spirit world to this world. It has an unsteady floor which sways and bends and over which [oe'r whose] our thoughts wander (our thoughts wonder on that unsteady floor), over the dark abyss that lies below.

So, the bridge of light connects the two worlds. Nevertheless, this world is located over an abyss. It is not clear, in fact, where "this world" is located, as the bridge of light seems to run horizontally up in the air and across the sky and the abyss is below it. That would make the sea and the abyss be more or less the same thing.