Mine Rescue Field Exam Taken from the 2014 Midwest Regional Mine Rescue Contest held in Wilmington, Illinois Progress Indicator: Question 3 of 30 3.  What is the origin of hydrogen sulfide? Choices: It is liberated by old timbers. It is liberated by burning explosives containing sulfur such as black powder or dynamite. It is liberated from pools of stagnant water in the mine. It is liberated when the methane content of air reaches five (5%0 percent or above.