There are two soughs there; the older 17th century one still flows into the back of the covered reservoir through a small opening, there is a concrete cap with a steel lid on it and a seriously rusted padlock a little to the south, put in when PDMHS attempted to dig the sough in the 1980s. From what I know of this, the sough was found to be badly collapsed in the Upper Millers Dale lava and the dig was abandoned.
The later early 18th century sough is at a slightly lower horizon but is completely collapsed, there's a line of small air shaft hillocks following the wall line to the north of the reservoir, the tail was probably around where the trees end as far as I could tell when I had a proper look at the site.
There's also a shaft on Wham Rake further south adjacent to the log washer base, which was full of rubbish when I looked at it with Steve Thompson ~15 years ago.