 (with slight wording changes so as to fit within the character limit for titles). All of the following predictions are about structures on Earth built by homo sapiens without time travel. He puts this at 20%. 

There was no time limit given in the post, but I've set the close date to the start of 2040 (else it could never resolve in the negative). 

Will resolve to Scott Alexander's judgement if he provides it; otherwise, I will use my judgement and reports in reliable sources to try to answer it (I will not bet in this market).