From Great Lakes Wiki
Large empty space or specially designed cargo hold used to store ballast water on board a ship. Ballast tanks can range in size from a cubic meter on small vessels, such as fishing boats, to thousands of cubic meters each on large commercial cargo ships. Most vessels have multiple ballast tanks and there are many different configurations based on ship type.
Ballast tanks on ships travelling through international waters can transfer invasive species large distances, although most ships are required by law to do Ballast Water Exchanges in the open ocean. This is a full flush of the tanks, replaced by cleaner ocean water.
Large commercial vessels are often required to use cleansing agents to kill off any unwanted inhabitants of ballast tanks, although this doesn't work perfectly.
