text: This is the site of Spro Gruve, an old feldspar mine active from about 1880 to after the first world war. It started as a day mine, but as day dug down they added a tunnel to the side as drainage and side entrance. It's no longer commercially viable, but you can still find minerals here and there.
  See http://www.oslofjorden.com/poi/spro_gruve_nesodden.html