Ballast tanks are large containers on ships or submarines that are filled with water or other heavy materials to help control the ship's balance and stability. Full definition
«The next step is to find out which stowaways will have the greatest chance to survive the journey in ballast tanks or on the ship hulls, and which are most likely to establish breeding populations after arriving in the Arctic. (sciencedaily.com)
The U.S. Coast Guard has instituted rules requiring the exchange of ballast water or sealing of ballast tanks on all ship traffic in the Lakes. (newworldencyclopedia.org)
Submarines let seawater into ballast tanks, when they are full the sub sinks. (science-sparks.com)