Its a heat exchanger.
The ground is cooler than the air in the hot summer.
Outside the desert you can use a water loop as well and/or run the loop through a water body.
Curious - what sort of trench depth are you digging, what size duct, for what sq ft / area home?
I am not sure yet. It's going to be a 20' circle essentially. I plan to go 3 feet or so. Deeper if needed. Though about using rocks to fill I the space of a tube if some sort. I am open to suggestions
I am curious how you went about choosing these dimensions
I found ceiling joists for free.
The depth I am guessing. I just make stuff up really and it tends to work out. 
Where do the ceiling joists come in?
Epic project. Looking forward to hearing updates
This abandoned trailer. We also got an epic shower upgrade from it. Major
quality of life improvement for free..
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Are you using earth bags? Or tires? As your outer walls a la an earthship?
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We are short on water for the time being. No plans to use water in it. Good idea though.
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