Cities that produce a lot of pollution will start to produce "dead tiles" within their city radius. Such tiles produce no resources. If pollution is left unchecked, eventually the game will give a warning that global disasters will occur. Disasters include change in climate, ozone deterioration, and global warming. In the case of global warming, the game informs the player that "ice caps have melted" and sea levels have risen. Tiles affected are turned into either coast or shallow water, and cities on tiles that become shallow water or coast are destroyed. The destruction of the ozone layer causes a large number of land tiles to become dead tiles. If a nation is appropriately technologically advanced, then that nation can repair dead tiles, but only at a significant cost of industrial production. The "Gaia Controller" wonder removes all pollution in the game but can be built only in the Diamond Age. Pollution is exacerbated by several city facilities such as factories and oil refineries. On the flip side, some facilities such as recycling plants and nuclear reactors will reduce the production of pollution. Additionally, certain events such as space launches and use of nuclear weapons will result in one-time additions of pollution each time that they occur. An initial setup feature is game play without pollution problems. While playing the game, the happiness level of the citizens must be maintained. If a city is far away from the capital, is overcrowded, polluted, overworked, starved or underpaid, the happiness level will drop with riots and revolts occurring. Cities also experience unhappiness during wartime, especially if they have just been conquered by another empire. Many terrorist units can decrease the happiness of an enemy city. For example, if a city is "infected", it will lose both population and happiness. If the happiness' level goes under 75, then the city is in danger of rioting. If a city riots, it does not produce any work during that turn. If happiness in cities continues to decline, revolution becomes a possibility. If that happens, the city's inhabitants become "barbarians", or change nationality to another country if another country has the Egalitarian Act Wonder. However, if the happiness of a city reaches very high levels, it "celebrates". Some buildings can increase happiness (e.g. temples, cathedrals, body transplants), and wonders can increase happiness (e.g. Immunity Chip, Ramayana). These can be built by any civilization who has acquired the technology to do so. They take a longer time to build than ordinary buildings or units of that age, but have a greater effect, and a cinematic is shown when the player builds one.