Swamps can support a Civilization though <l:8,0>farming<e> is impossible. Movement through the ooze is reduced to a crawl unless <l:8,19>roads<e> are built, which is costly in a swamp. The same ooze, however, provides a natural defensive bonus. <l:6,25>Rivers<e> greatly enhance this terrain. Swamps are also unstable and cities built on them are prone to sink in a case of a severe global warming.