听力原文： An explosion in a Pennsylvania coal mine killed one worker Monday, but five other miners escaped. The blast went off 2,300 feet underground at the R&D coal mine in Schuylkill County. Some officials say it appears a blast went off in the anthracite mine when workers detonated the explosives, but that's still under investigation. The process right now is to continue the interviews and then get back down underground and complete the inspections and in that way we can complete the investigation as to what happened. The brother-in-law of the miner killed says 43-year-old Dlae Reightler worked at the mine since he was a teenager and that he dreamed of moving his wife and four children, setting up an auto-repair business. He did it all his life. He did that and worked on cars in his garage backyard. He enjoyed doing that stuff. At the mine, the accident knocked out the ventilation system, making it too dangerous for investigators to go down into the mine shaft. Regulators ordered the facility closed until the investigation is complete. The area has the nation's only deposits of anthracite, a type of hard, clean burning coal. A June inspection of the site turned up no violations and violations detected in August were fixed on site. The investigation was dangerous because ______.