Ocean-going ships load up with water in bilge holds using the extra mass to keep them stable while they ply their way to their destination port. More ballast is used when ship are not fully loaded with cargo and this water is then pumped back into the sea when the ship takes on new cargo. Many problems can result if discharged ballast water contains pollutants or living organisms that can potentially have negative effects on local marine life at the destination port. (Source: UNESCO)