|This article is about an older version of DF.|
Underground rivers occur in almost every region with a mountain biome, similar to the appearance patterns of adamantine. Upon discovering an underground river, subterranean plants will begin growing in suitable locations (on muddy floors, and on soil floors with a soil wall nearby beneath them).
On maps without a river, brook, any underground soil or aboveground plants to gather (so you can therefore get their seeds and begin farming aboveground) these can be a blessing since they provide water for muddying tiles (allowing both farming and plant gathering underground), but on others they can be a hindrance, because you have to dig around them.
The underground river is populated with frogmen, lizardmen, snakemen, olmmen, cave crocodiles, giant toads, giant olms, and mud men. These can interrupt the work of any dwarf within line of sight, and can thus be a real nuisance.
Cave spiders are a type of vermin that occur near underground rivers. They leave small webs strewn around, which can be collected for silk weaving. Fisherdwarves may also be able to catch cave fish and cave lobsters from the river itself (though not from other water tiles nearby), and olms may occasionally be found walking around (and potentially getting caught in cave spider webs).
The water sources for an underground river, if they are present on your map, are waterfall tiles "from nowhere". Correspondingly, an underground river will terminate in a waterfall that drops into a chasm at the bottom of the map.
Underground rivers are 2 to 5 tiles wide and while they often flow in a straight line, they can twist and turn across the entire map, spanning multiple levels.