Artificial light at night has spread faster than almost any other kind of pollution, and unlike most it is invisible in the daytime and leaves no residue. Its effects are nonetheless measurable. Insects gather at lamps until they die of exhaustion, migrating birds are drawn off course by lit buildings, and turtle hatchlings move towards streetlights instead of towards the sea. Because each individual lamp seems harmless, the damage is done by the total, for which nobody is responsible. Ecologists point out that darkness, unlike clean air, has never been treated as a resource worth protecting.