If you’re an ocean creature with a hard shell—like a sea urchin, a hermit crab, or a coral polyp—you prefer ocean water with a pH of about 8.2. This chemistry makes it easy to assemble your armor from carbon-based building blocks dissolved in the ocean. Since the beginning of the industrial age, though, the ocean has been absorbing the greenhouse gas carbon dioxide from the air. The increase in carbon dioxide has made the ocean’s pH more acidic, dropping to 8.05 on average. Biologists like Gretchen Hofmann are realizing that this tiny change is hampering the development of hard-shelled marine creatures, leaving them more vulnerable to environmental stressors. To learn more, Hofmann’s team has recreated an acidic ocean in a lab at the University of California, Santa Barbara, and is testing how the change affects sea urchins.