Something similar to what my father found at Wet Earth Colliery, near St Helens in Lancashire, UK. A complete scheme of underground tunnels, an inverted 20-foot diameter syphon under the River Irwell, with different tailraces, weirs and other water control mechanisms that had been totally forgotten. There was also a large underground pumping station with sluice controlled waterwheels (or turbines depending on the period) used to pump water out of the Colliery. This was recently shown on Fred Dibnah's prog on BBC2 (alas, without credit to my Dad who found it with his partner Geoffrey Banks). Quite an amazing piece of industrial history - which at the time of it's creation was unique and hailed as a superb piece of engineering considered "of national importance", establishing the reputation of James Brindley as a skilled Civil Engineer.

Now it's not even marked on the map.

PS Do a Google search for "James Brindley" and "Wet Earth Colliery" and you turn up some interesting results. Have a look at this page for example.