Underground river

From DwarfFortressWiki

Underground rivers occur in every region with a mountain biome, just like adamantine.

In maps without any underground soil or aboveground herbs to harvest (so you can therefore get their seeds and begin farming aboveground) these can be a blessing, but on others it can be a hindrance.

The underground river is commonly filled with aggressive creatures such as Snakemen, Olmmen, Giant Olms and Cave Crocodiles. These can interrupt the work of any dwarf within line of sight, and can thus be a real nuisance.