For four billion years, the moon's polar craters have held their darkness like a secret, trapping water ice in temperatures colder than the outer solar system. Scientists have now measured that secret against humanity's grandest ambitions, and found a sobering asymmetry: the ice exists, but not in quantities sufficient to sustain the million-person cities that space entrepreneurs have imagined. What the numbers do support — small, careful, patient settlements — speaks less to the limits of the moon than to the perennial human tendency to dream at a scale that outpaces the resources at hand.