football fields.

I thought you folks would be really interested in this, but no one replied to the first post:

Meeting the challenge to provide the Raiders with a real grass field in the desert, the facility also features a retractable field system. Weighing nearly 20 million pounds, the playing field is housed in a movable four-foot-deep “tray” and moves on 540 electrically-powered wheels across 13 rails, which allows the natural grass field to move outdoors for sunlight and water. This feature provides the stadium with the versatility of offering both natural grass and artificial turf playing surfaces.

I havent' figured out where it goes when they retract it. It's not small, is it? It's 4-feed deep and as big as a football field.

Then t here is the issue of 'rolling up the turf'. See the top here.

Plus it seems like a lot of money when just one playing surface would do. Don't they only have one at most stadiums?

When it is retracted, it is inside the stadium. When it is extended it is outside the stadium getting desert sunlight and wasting scarce water resources.

It is not the only movable grass football field, the Arizona Cardinals also play in the State Farm stadium with retractable turf.