Humanity's long-held dream of cities on the moon encounters a quiet but unyielding boundary: water. New research reveals that the moon's polar ice deposits, finite and fragile, could not sustain a million-person settlement beyond a single century — a finding that asks us to reckon with the difference between what we can imagine and what the cosmos will actually allow. The vision of lunar civilization does not disappear, but it must now be rebuilt on more honest foundations.