“It’s a desert for a reason,” said Lynn Fenstermaker, a research professor at Nevada’s Desert Research Institute. “Flooding the desert and then keeping the water there, in an already water-poor area with all the evaporation, is hard to imagine.”

Y Combinator doesn’t deny the magnitude of the challenge. “Economies of scale as well as breakthroughs in material science and construction technology will all be necessary for success,” its proposal says.

Y Combinator pegs the price tag at $50 trillion. That’s roughly half the entire globe’s economic productivity for a year. Altman said in an interview that the cost for any solution will need to drop into the billions to become more realistic. “You can do a lot of things that require spending more money than you will ever be able to get,” Altman said, “and it just doesn’t come.” Brought to a more realistic price, he believes that governments will pay.