sentence: Some rivers flow underground through caves.
sentences_after: Underground rivers form in places where there are lots of cracks in the rocks above, so that in rainy weather, the water runs downs and collects in small underground streams. In other places, where there are caves, the small underground streams run together to form a river. The river can sometimes run through deep wide underground caverns. While many underground rivers flow gently, some underground rivers flow fast and have rapids, particularly after heavy rain. Many underground rivers flow out through a cave mouth to become an ordinary river.
concept_name: river
headings: River About rivers Underground rivers
categories: Basic English 850 words Biomes