I believe, that the square north of Highway 8 in Minnesota, I marked with osm.org/browse/way/184791849 is the grounding electrode of HVDC Square Butte, described on http://web.archive.org/web/20051115122807/http://www.transmission.bpa.gov/cigresc14/Compendium/SQUARE.htm and not a quarry. Can a mapper in this area confirm my assumption? Nevertheless, the grounding electrode of Square Butte must be in this area. When you have a close look at the powerline, which uses the pylon osm.org/browse/node/1947051066 you discover, that the line has west of this pylon 3 and east of this pylon 4 conductors, whereby the fourth conductor is the ground return of Square return and runs directly to the converter of HVDC Square Butte at Adolph HVDC Converter Station.