Scientists are concerned about possible diseases that can be spread from the ballast water of ships. Ballast water is held in tanks or in cargo areas of ships. It helps keep the ship floating when the ship is not carrying enough heavy cargo. It is used to keep the ship balanced when it is sailing in the open ocean. Large ships usually carry millions of tons of ballast water. The ship later releases the water at ports of call. Almost 90 million tons of foreign ballast water are released into ports in the United States each year. Ballast water can contain millions of large and small organisms. Some of these organisms may threaten life in the oceans or may be harmful to human health.