Poppy Harrington has spent her life studying the tides, preserving harbor records, and trusting the measurements that keep her coastal world in order. But when the mercury column in her grandmother's archive plunges without warning, Poppy follows a strange, singing copper cable into the clouds-and discovers a colossal floating conservatory twelve thousand feet above the sea.Inside the Sky Aquarium, impossible spheres of water drift through a cathedral of brass and glass, each containing extraordinary organisms that seem to belong to no world on Earth. Among them is Aethel, a mysterious, finless creature that swims effortlessly through the air. Drawn to its gentle intelligence, Poppy becomes determined to understand where it came from and why it is being kept inside a floating sanctuary.Her investigation reveals a startling truth. The creatures are not ordinary specimens collected for scientific curiosity. They are living beings whose natural worlds have yet to come into existence, suspended in an artificial refuge until their distant horizons can form. Their survival depends on conditions that the familiar ocean below cannot provide. What appears to be protection may also be a prison.The conservatory's stern Keeper, Elias Hawthorne, believes that strict control is the only way to keep these fragile lives from being destroyed. Haunted by the disappearance of his daughter, Maeve, he has spent years measuring pressure, regulating the atmosphere, and preventing anything in his care from falling into danger. Yet Poppy begins to uncover evidence that Maeve understood something Elias has refused to accept: preserving life is not always the same as allowing it to live.As violent atmospheric disturbances threaten the vessel, Poppy must look beyond the familiar rules of tides, pressure, and gravity. With the help of unexpected allies and the secrets left behind by Maeve, she discovers that the creatures' future depends on something far less measurable than air density or water salinity-the power of longing, imagination, and the desire to reach a world not yet seen.But freeing Aethel means risking the fragile sanctuary that has kept it alive. Poppy must decide whether safety is worth a lifetime of confinement, and whether the unknown horizon beyond the sky is a destination to fear or a possibility worth everything.Poppy and the Sky Aquarium is an atmospheric fantasy adventure filled with extraordinary creatures, intricate skybound machinery, hidden histories, and breathtaking discoveries. Exploring the tension between protection and freedom, grief and hope, it asks a profound question: when the world we know cannot offer someone a home, do we keep them safe-or help them find the world where they truly belong?