Not to be confused with the ancient track also known as the Ridgeway, this is a public path along the top of London's Southern Outfall sewer where it runs inside an embankment for about 5km from Plumstead to the sewage treatment works at Crossness. See
Link for the walking route, and
Link for the sewer itself.
After several years when it became overgrown and nearly impassable in places, Greenwich council upgraded their section of the Ridgeway in 2015. The Bexley section remains unimproved at the time of writing.
See other images of Plumstead-Crossness Ridgeway