Much of the area is dotted with giant pillars of coal, left standing in a checkerboard pattern to hold up the mountain above the Eagle coal seam and prevent the mine's ceiling from collapsing.
A map plotting where rescuers found the bodies of victims shows the blast spread north, east, south and west simultaneously, and repeatedly turned 90-degree corners when it encountered coal pillars and passageways.
Experts say that's precisely how explosions behave in the confined space of a mine -- if fuel is available.
"The best way to describe it, it follows the source of fuel," said Michael Sapko, who studies coal mine explosions at the National Institute for Occupational Safety and Health, the government's chief mine safety research organization.
Mines use complex ventilation systems powered by enormous fans to push fresh air in and methane out. Experts say the effect is usually a breeze ranging from mild to stiff.
Since the 1920s, mines have also been required to use pulverized rock to cover coal dust with a protective layer of inert material.
In a properly dusted coal mine, Sapko said, an explosion can be stopped almost in its tracks. But research by Sapko's agency shows even the thinnest coating of coal dust is dangerous enough to instantly transform a small explosion into a giant one.
"Once it gets in the air, and if there's an ignition source present, it's uncontrollable at that time," Sapko said.
Months of examination reports written by Massey employees and years of violations cited by federal inspectors indicate Upper Big Branch miners routinely struggled to keep the mine dusted.
MSHA records show inspectors have slapped Massey with at least 475 citations, including not having enough rock dust to prevent explosions and allowing combustible materials pile up at Upper Big Branch since it opened in 1994.
One of the most serious came less than a month before the explosion. During a routine inspection, MSHA scooped up eight samples and sent them to a lab to make sure they contained enough rock dust.
Lab tests determined one of the samples contained less noncombustible material than required by law, according to a citation issued eight days after the explosion.
MSHA rated the mid-March dust violation "significant and substantial," saying it was "reasonably" likely to cause a fatal accident if not corrected.
"This standard/condition has been cited 8 times in the last two years," an unnamed MSHA employee wrote as justification on a form dated April 13.
The samples came from a spot called Headgate 22, an area that was being prepped as the next section to be mined using Massey's longwall mining machine. It was about three miles from the nearest entrance.
"Needs dusted," says a report from that area earlier in the day April 5.
The problem was reported, but there's no indication it was fixed.
The final notation in the record book for Headgate 22 notes small concentrations of methane in four locations and the same problem with one of them: "Needs Dusted."