Many cities — probably most, even, except those built in desert environments — are built on top of streams, creeks, marshes, wetlands, and so on. As the cities develop, those watereways are diverted into storm drains and other types of channels to allow construction on top of their former surface pathways.
But what if those now-underground streams were accessible to city dwellers?
…We had a lantern to pierce the cellar darkness and fifteen feet below I clearly saw the stream bubbling and pushing about, five feet wide and upon its either side, dark green mossed rocks. This lively riverlet was revealed to us exactly as it must have appeared to a Manhattan Indian many years ago… [quoting from the New York Times by way of the blog urbablurb]