Solid cement. The football field is removed outside leaving a cement slab.
Concrete... the grass is on rollers and it rolls out of the building.. it's very cool YouTube the show Build it Bigger
Concrete.
The field rolls outside so that it can get some sun. Cement prevails underneath.
Concrete. I've ridden a motorcycle through it and drive a semi into Univ Phoenix Stadium. The field rolls in from the south east to the northwest on rollers. The sidelines are built up to match the height is the field tray. Cool stuff!!
Earth
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