An explosion at a coal mine in the town of Montcoal, W.Va., has killed 25 people, and become the worst mining disaster in the U.S. since 1984. Although the tragedy is still unfolding, it is already becoming clear that this particular mine had failed several prior safety checks and questions are being asked about the security of the industry in the wake of the tragedy. A brief guide to how the disaster unfolded:

What happened?
At around 3 p.m. on Monday, a chamber approximately 1,000 feet below ground in the Upper Big Branch coal mine exploded, killing 25 workers and leaving four missing. Two others were seriously injured.

What caused the explosion?
We won't know precisely what happened until after an official investigation, but it appears combustible methane gas venting into the mine may be to blame.

How powerful was it?
It could be heard and felt for miles around. Miners working underground at another site seven miles away said it felt like being "in the middle of a tornado" as dust and wind surged through the tunnels.

How many people were in the mine at the time?
The mine's owner, Massey Energy, employs around 200 miners at Upper Big Branch, but not all of them would have been working at the time.

Is it possible to survive a mine explosion and/or collapse?
Yes. Every mine has airtight chambers containing enough food, water, and oxygen to let stranded miners survive for four days. After the Sago Mine explosion in 2006 one worker, Randal McCloy Jr., survived after 40 hours trapped underground. But one of the Upper Big Branch's two safety chambers were found to be empty after Monday's explosion, and the rescue operation had to be suspended before the other could be checked.

Why was the rescue operation suspended?
It was feared that toxic gases leaking into the mine would cause another explosion. Until bore holes are drilled 1,200 feet into the ground to ventilate the mine, it is too dangerous to launch a full-scale effort. According to CNN, this drilling operation could take up to two days. But safety officials say it is very unlikely any more survivors will be found.

What safety issues has the mine had in the past?
In the past year Performance Coal, the subsidiary of Massey that runs the mine, has been fined more than $382,000 for "repeated serious violations." These include failing to follow ventilation plans, letting combustible coal build up, and having improper firefighting equipment. The firm was fined for ventilation problems three times last month, most recently on March 30.

Are mine explosions regular occurrences?
It has only been four years since the last fatal mine explosion in the U.S., which also occurred in West Virginia. The Sago disaster killed 12 workers, and prompted a review of safety and health systems in American mines. That this "regulatory overhaul" failed to prevent another explosion will be one of the main topics of discussion as the fallout of the disaster continues. "We just can't allow this to continue," West Virginia Gov. Joe Machin told FOX News.

What can be done about it?
The federal Mine Safety and Health Administration needs to improve, according to an NPR report. A former MSHA chief told the network that most coal mines don't have operational communication systems, and many fail to take basic precautions against the type of explosions that occurred yesterday. Although the MSHA increased the number of mine inspectors by 26 percent after the Sago explosion in 2006, many still have inadequate training, reported FOX News.

Sources: BBC News, FOX News, ABC, NPR, CNN, LA Times,