What are the underground rivers?

Rivers run underground all over the world. These occur naturally in cave systems, and have been known since antiquity, as evidenced in legendary myths of underground waterways such as the River Styx which forms the boundary between Earth and Hades. But this is about subterranean rivers that were once open to the sunlight and were buried by human hands (or heavy machinery). This happens when cities are built over top, when wetlands are drained, and when existing waterways are consolidated and hidden to give people and their infrastructures more room to grow or move. In some of those places, people don't even realize there's a river running underneath their homes.