Pollution is usually perceived to be man-made, but in fact the atmosphere and waters of the earth can also be contaminated as a result of natural events. Volcanoes can emit huge clouds of gases and ash, and flooded rivers typically carry high loads of silt and organic debris. Nor is such naturally-occurring pollution always associated with extreme events (which happen rarely but cause major impacts) such as volcanic eruptions. For example, soils developed where gossan (the oxidized crust on an ore body) outcrops at the ground surface may contain very high levels of the metals found in the are itself - levels that would be labelled highly contaminated if they occurred in waste deposits from a mining operation. Nevertheless, it is obvious that urbanisation and industrialisation increasingly produce a range and quantity of contaminants which the natural environment cannot absorb without being degraded and which can have deleterious effects on human beings. Because the problem has seemed to be so serious, most countries have legislation and controlling authorities designed to minimise pollution and its impacts.