I rapped my knuckles on the galvanized aluminum exterior to make sure it wasn’t some sort of desert mirage. It was solid: $400 million worth of concrete, electronics, and steel assembled to rotate a thick rubber belt along roughly 66,000 metal rollers. Something on the order of 13 million tons of sand can be carried the entire length of the machine—42 miles—each year. If all of that were used to build sandcastles, you could have a couple of dozen the size of Buckingham Palace, with more than enough left over for a Taj Mahal.But this sand isn’t for beachside amusement. It’s for fracking. When drillers crack open subterranean rock in the Permian Basin to allow oil to flow out, sand rides alongside the injected fluid to…