It is a permanent and densely inhabited area with officially defined limits, whose members predominantly work on non-agricultural jobs. In general, cities have comprehensive systems for housing, transportation, sanitation, utilities, land use, manufacturing, and communication. Many cities also have government agencies that are responsible for enforcing laws and regulations. Cities are important centers of activity for their regions and sometimes other countries as well. They often contain major airports, seaports, and highways, which make them useful for trade and travel. Cities also provide many other benefits, such as social interaction with neighbors and friends, cultural events, and recreational opportunities. In fact, most people in industrialized countries live in cities, so the existence of cities is necessary for human survival. However, not all people need to live in cities to be able to meet their needs. Some people can survive very well without ever going beyond the reach of civilization. Cities are known for their wealth and luxury goods, but this has always been the case since humans began living together. Before the advent of modern industry, cities were home to wealthy people because they provided easy access to resources that were difficult or impossible to find elsewhere. For example, before oil spills and pollution became problems, oil was found only in cities because there were no other places where it could be found in large quantities.