Well, the explosion at Massey Energy’s Upper Big Branch Mine in Raleigh County, W.Va., already ranks as the worst mining disaster since 27 workers were killed in a fire at Utah’s Wilberg Mine in December 1984.

And my coworker Gary Harki is reporting from the scene that federal and state officials fear the death toll could go higher — with four miners still unaccounted for and MSHA’s Kevin Stricklin calling their situation “dire.”

So what happens now?

The top thing to be watching is the rescue efforts, which were temporarily halted early this morning because of dangerous gas levels in the mine. Rescue crews are starting the process of drilling boreholes down into the mine, to help try to stabilize the atmosphere there and make it safe for rescue teams to resume their search.

As I reported earlier, we got somewhat conflicting accounts on this effort early this morning in a briefing by Stricklin and Ron Wooten, West Virginia’s mine safety director — with Wooten sounding much more optimistic than Stricklin about the four miners’ chances.

Gov. Joe Manchin, ever the optimist — but also a veteran of these situations, having lost his own uncle in the 1968 Farmington disaster — had this to say early today:

We are still in that rescue operation mode. With that being said, three holes have to be drilled. The best I can tell you is that it’s going to be a very long day.

Meanwhile, the other things to watch through the course of today and beyond:

— Massey’s safety record: This is sure to get lots of attention, given the company’s long history of problems (subscription required), the criminal prosecution following the Aracoma Mine fire, and MSHA records that indicate there were growing concerns at the Upper Big Branch Mine.

— What exactly caused this explosion: There were early reports — not officially confirmed by government officials — that the explosion may have come from inside or around a sealed area of the Upper Big Branch Mine. If that turns out to be true, it makes this disaster terribly similar to the Sago Mine disaster and the Kentucky Darby disaster, and raises major safety questions about sealed areas in mines across the coalfields.

— Along with that, what kind of investigation are we going to have: MSHA has authority under federal law to investigate the disaster through a public hearing, but the agency has always hesitated to do that, opting instead for closed-door interviews. After Sago, Gov. Manchin brought in former MSHA chief Davitt McAteer as a special investigator and ordered a week-long public hearing into that disaster. What will Manchin and MSHA do now, especially given that Obama MSHA chief Joe Main has been an advocate of his agency being more transparent?

— Finally, this disaster was a huge test of whether the MINER Act reforms went far enough: There was hope late last night that the miners had reached a cache of self-contained self-rescuers and perhaps then made their way to one of two mine rescue chambers — two important pieces of rescue equipment that were added because of the MINER Act and state laws passed after Sago, Aracoma and Darby. And Congressman Rahall praised what he said was a very fast response by mine officials and government agencies, another big difference from the 2006 disasters.

On the other hand, something that jumped out at me was when Stricklin explained during a briefing this morning that the new communications and tracking system had only been “partially installed” at the Upper Big Branch Mine.

In a story a few weeks back, I explained that only one in 10 U.S. coal mines had so far met the communications and tracking requirements of the MINER Act.  That story took some heat from Randy Harris, a consultant for the West Virginia Office of Miners Health, Safety and Training. Randy felt I was unfair to the industry in West Virginia, where companies have for the most part moved to comply with a separate state law requiring communications and tracking equipment.

Sadly, though, the Upper Big Branch disaster indicates the importance of a difference between the West Virginia law and the MSHA regulations that grew out of the MINER Act … specifically,  West Virginia rules require companies to track whether miners have entered a working section of the mine, but not exactly where they are on the section. By contrast, MSHA requires more specific tracking of where the miners are on the section — information that might be helpful right about now at the Upper Big Branch Mine.

Ron Wooten, director of mine safety in West Virginia, explained the state’s law during a briefing this morning:

West Virginia law requires that we know when people are momving onto a section. It doesn’t require that we track them on a section.

And Stricklin explained the MSHA requirement, and how it not being met yet by Massey Energy could have been a problem:

We know how many people are in that area, but we don’t know their exact location.