for the cement slurry into hollow area under earth mantle 灌入地幔之下空洞的水泥浆
as you have oil digging facilities, just reverse, and pump the cement slurry (density 50%) into the hollow. then measure the depth between the water surface and mantle. underground river is good for the soil, and will change desert to soil.
it is HUGE investment, which requires more than a lot LARGE amount of the cement slurry into the hollow area.
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