There's a quarry in the Chicago area, quite close to the city and visible from the interstate, that's over 100 yards deep: https://en.wikipedia.org/wiki/Thornto

There's a quarry in the Chicago area, quite close to the city and visible from the interstate, that's over 100 yards deep: https://en.wikipedia.org/wiki/Thornton_Quarry#/media/File:Thornton_Quarry2.jpg https://en.wikipedia.org/wiki/Thornton_Quarry#/media/File:Th.... Digging such a deep hole isn't ridiculous at all. Also, as the video shows, landfills usually rise a significant height above the surrounding land (and are converted into a hilly park when finished).