Underground Rivers is a worldgen mod that adds underground rivers to your world.
Tectonic has underground rivers as a feature, but this mod adds them to vanilla terrain with no other changes. They're a complete rewrite from the ground up, meaning there's better entrances and more variation then ever before!
Underground Rivers relies on the Lithostitched mod for compatibility purposes, making the mod work with other worldgen mods like Biomes O Plenty, Nature's Spirit, and even Terralith!
