text: I can not find any references to an underground river.  There are no rivers here on the OS map from 1912, https://maps.nls.uk/view/114581590.  The map references windmill but no water mills.  The river is drawn as going under the ridge that mount road runs along, which suggests a lot of digging would have been required.  Also the catchment area is presumable not very large.
  So I suspect this underground river is a mistake and should be removed.