The meaning of a bold-faced sentence

The meaning of a bold-faced sentence

The passage below is from Fathoms: The World in the Whale Hardcover by Rebecca Giggs.

I once thought of the gyres as producing artificial islands; aggregations of fisheries’ junk, jettisoned cargo, debris drawn into the ocean by tsunamis and high winds, and trash flushed off the land by stormwater. I imagined a pell-mell of spoilage — pontoons of human language atop the languageless sea, rasping softly, Coca-Cola, Sprite, Gatorade, Schweppes, Sunkist, Mountain Dew, Nestlé. The surface scythed, maybe, by a wingtip from Malaysia Airlines MH370—crashed into the ocean and still unaccounted for. Or even: a terrible, sucking funnel, at the tapered root of which I shut my eyes and saw dark things, smoothed into jostling, anonymous spheres by the constant, spinning motion of the gyre. Not a void in the seascape, then, but a ghastly commons, a humanmade middleness. A buoyant midden.

In this passage I have problems with the bold-faced sentence. Since this is happening in the author’s imagination, it’s subtle and mysterious.

Let me tell you what I thought reading this passage.

First, ‘Not a void in the seascape’

Right, since there is so much human garbage out in the ocean. There’s no empty space in the sea. (Am I right?)

Second, ‘then, but a ghastly commons’

What is ‘commons’? I looked up Marriam-Webster and got one that seems to fit in this context.

5: b: a public open area in a municipality

1. A food and jazz festival will be held at the town common.

(Am I right?)

Last and hardest, ‘a humanmade middleness’. What’s the meaning of ‘middleness’? In dictionaries there’s only one meaning such as: The fact or condition of being middle. But this meaning doesn’t do much for me to grasp this sentence.

Can you help me?

Thanks in advance.