At first, they look like stars. I see them as I gaze upward at the ceiling of a flooded, pitch-black cave—hundreds of blue pinpricks. As my eyes habituate to the darkness, more and more of them resolve, and I see that they are brighter and more densely packed than any starry field. And unlike the night sky, these lights don’t appear as a flat canvas, but as a textured one. Some are clearly closer to us than others and they move relative to each other, so the whole tableau seems to undulate gently as our boat sails beneath it. These lights are not astrological, but entomological. They are produced by insects called glowworms.

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