The awesome power of science fiction's alien megastructures

The imaginary constructions of science fiction fill us with awe at their alien vastness. Which have you explored, and what was the most overwhelming?

Sci-fi fans call it "sensawunda", that awe and amazement that the best science fiction stories can inspire in us. The entire world felt it recently when scientists declared that observations of a distant star might have revealed an alien megastructure. Did inhabitants of the KIC 8462852 star system encase their sun in solar panels to harvest energy? Or was this our generation's canals on Mars moment? The sensawunda effect is so powerful that, even with scant real evidence, we are swept into believing.

The unlikely cause of KIC 8462852's strange emanations is speculated to be a partially constructed Dyson sphere. Imagine the Death Star from Return of the Jedi, in its partially constructed state, but on a scale large enough to swallow a star. Engineering on this scale is as far beyond human capacity today as building a skyscraper was to our cave-dwelling ancestors. Imagine the power of of a civilisation that can capture a star. Then imagine the drama that might stop the construction work partway, a very real war among the stars. There, that is sensawunda!