The first edition O.S. map shows a feature called "Fish Pond" in this location - check out the maps, I've done a comparison of current and old maps.
Please click here for the original source - Information from The Modern Antiquarian article.
The feature seems to consist of a banked earthwork that has been cut through by a stream producing a clear section. From the stream bed to the top of the bank is estimated to be 20 to 30 ft.
The section clearly shows a solid core to the bank a gravel infill at the sides that is topped off by peat. The age of the site is difficult to estimate, but while walking around the top of the bank "bog oak" and other timbers have been reportedly seen emerging from the eroding peat.
So, have a look for the remnants, including the two large square bases that were part of a structure of some sort... and see what else you can find. Due to the remoteness of this area, it's very underexplored, and there's a lot more to uncover about this mysterious pond!
Thanks to user 'Rivington Pike' for posting the original article on The Modern Antiquarian. I wouldn't have known about this spot without their input.