It is very difficult to clean up liquid and gas wastes.

The easiest to clean up is sewage because it is always kept in pipes and people know that if they do not clean it up then there will be problems with disease.

Oil and other spills are usually accidents, but they can be disastrous.

The most important pollution source is, however, our cars, planes, homes and power stations. The world's governments cannot yet even agree on how to cut this pollution and stop the result: global warming.