Contour Ring Hunting.. yes, it's a thing (well at least on Dartefacts anyway!) This activity requires the visiting of ALL Contour Rings located within the Dartmoor National Park. A contour ring can be a small mound, a huge plateau or even a pit or depression on the Ordnance Survey 1:25k map. It must be a contour 'loop' that contains no other contour loops. Sometimes it can be surprising to find that there is no mound present, the height of the terrain just happened to clip a contour line. The funniest are the ones where you're expecting a large mound and you find it is a depression or pit. At the moment I am uncertain how many of these contour rings there are within the National Park.. it will certainly take some time (and eye-strain) inspecting every one of those 1044 km squares! The Dartefacts Map (below) displays 1:25k map features for logged-in Registered Users which will show you every contour ring listed, whereas the standard 1:50k mapping does not.
Do not assume that a Contour Ring listed is legally accessible.