The Maidenhead pits had been discovered and opened in 1821 and had many shafts sunk into the property ranging from 150 to 700 feet deep. Some explosions occurred in these mines prior to 1839. The two major and deepest shafts were 700 feet deep (the 1839 explosion happened in this shaft) and 600 feet deep (this shaft was completed in 1840). The coal from these mines was about 36 feet in thickness and in 1841 the Maidenhead mines were estimated to produce 2 million bushels of coal per year (at the rate of 1 bushel = 80 pounds, this would be 80,000 tons per year and about 219 tons a day).