The sand can be re-used, of course, but eventually the grains become too rounded to hold impressions well. The open areas among the wooded hills are coal strip mines in eastern Kentucky. Although the scene is 2,000 feet underground, it's still 10,000 feet above sea level. Older mines had rails, but modern mines tend to use diesel earth movers. To do that, you need adequate ventilation, and most large mines have a noticeable wind blowing through them.
Some mines move more air, pound for pound, than they do ore.
All mining is like a military retreat - you pull back with careful planning all the way. The mining at Climax was once done by stoping - caving in the ore from below. The mine is sitting right on the Continental Divide at 11,000 feet elevation. This picture was taken in May . Later on, mining was carried on by surface mining.