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As of the 2010 census, the United States consists of 11,078,300 census blocks. Of them, 4,871,270 blocks totaling 4.61 million square kilometers were reported to have no population living inside them.
This guy made a map with dark green areas representing blocks where the reported population equaled zero.
I’ve changed the green to the same hue as the ocean, which I think makes it easier to visualise the occupied land; a water world type scenario if you will.