Definitely a mineshaft. There are thousands of them around all unlisted or poorly recorded. Stoke on Trent has very shallow coalmines under part of it's streets. I remember seeing one core come up with part of a pit prop and a neatly drilled section of railway line, back in the nineties. I have also nearly lost rigs down un-mapped bell-pits in the welsh valleys, and also hunted for concrete capping blocks over minseshafts with a 360 excavator. A backhoe is a bit too close and personal if there is uncertainty as to the integrity of the cap. In north Wales, in the old copper mines, shafts would often be covered over with tree branches and waste material placed on top.This is fine when recent, but after a hundred years the adits become lethal to traverse.
Even in rural Ireland, out in west Cork, mines rear their ugly heads. The road from Durrus to Balledehob had a collapse several years ago due to subsidence of old copper/barytes mine workings.