Soccer ball building foundation - GIF

Soccer ball building foundation.

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Previously:

Excavator playing goalie in giant soccer game GIF
Making a wooden soccer ball - GIF
Marking toy soccer balls - GIF
Railway that intersects soccer field - photo
Soccer fans bang out car dent - GIF
Soccer training in the 1950s - GIF
Turning soccer balls inside out - GIF

Weight saving? - or just wanted to add a little 'bounce' to the floor...

Probably weight saving and concrete saving. It looks like a good idea as long as the cement is at a reasonable strength level.