The Greenpeace vessel Esperanza unloads one of the nine enclosures known as mesocosms (literally “midsize worlds”) used in the acidification experiments. Developed by biological oceanographer Ulf Riebesell of the Leibniz Institute of Marine Sciences in Germany, the mesocosms consist of a buoyant frame and a 65-foot-long polyurethane bag that encloses plankton and other small marine organisms.