Most often when we think about exercise we think aerobic. That is in part because of the high energy classes available at most gyms. Aerobic exercise is not confined to those classes, however. For most people, low to moderate exercise or exertion is generally aerobic.
The biggest iron mine. The worlds biggest open pit iron mine can be found near Hibbing, Minnesota. At its maximum, the area covered by the Hull Rust open pit mine is five miles (8km) long, 2 miles (3,2km) wide and 535 feet (180m) deep. It looks like a man-made Grand Canyon of over 2,000 acres (8,1kmÂ²).
But unlike in 2012 when Wyoming was home to nine of the US' ten biggest coal mines, the Cowboy State had a bit more competition last year. North Dakota's Freedom mine has taken the spot of the US' tenth biggest coal producer, as Wyoming's Caballo slid off the top-ten chart. All production figures are in metric tonnes. Here are the top-ten biggest US coal mines in 2013 in terms of production.