That spot, The Washington Post's Chico Harlan explains, was also where North Korea conducted its two prior tests, in 2006 and 2009.
And there are some pretty telling clues in the neighborhood, all on full display on Google Maps. First, the earthquake site is awfully close to a road labeled on Google Maps as "Nuclear Test Rd." Here's the map:
Now, if you actually drove on this road, "Nuclear Test Rd" is almost certainly not the name you would see on street signs; the North Korea road names largely come from user-provided data within the country. If the road doesn't have a formal name and you were a North Korean activist hoping to bring some transparency to the world's most opaque nation, you might give it a name that conveys a bit more information.
Sure enough, zoom in on the road and, about one kilometer south of where it ends, you'll see a cluster of gray buildings labeled "nuclear test facility." (Clicking on the facility brings up a photo of a smoking Kim Jong Eun, a bit of user-generated humor.) Any nuclear test would be underground; presumably, the point where "Nuclear Test Rd" disappears is where the actual road goes beneath the mountains. Here's the facility:
Zoom out a bit and the Hwasong Gulag comes into view, maybe a dozen miles to the east. The concentration camp, also known as Camp 16, sprawls over several miles. Joshua Stanton, who runs the One Free Korea blog, posted a detailed satellite-view tour of the camp. Here it is with the nuclear test and nuclear facility, marked by the green arrow and red button, in view:
Here, just below, is Stanton's rough annotation of the suspected bare-bones houses across Camp 16, the locations where prisoners -- sometimes full families -- would live. His map shows just how enormous the camp is, and how close to the nuclear test facility to the west:
And here's an image of a small collection of buildings with some of the maybe-prisoners actually captured on satellite:
"See the people in green at the bottom center? They might be guards, but it’s more likely they’re prisoners," Stanton wrote. Look for the small grid of green dots, just below the southernmost red-roofed building. "At about midday on June 10, 2008, a satellite captured this image of them."
Camp 16, even more so than North Korea's other work camps for political prisoners, is shrouded in mystery and death. "Prisoners are never released from Camp 16, dead or alive," Stanton wrote. "No witness has ever emerged to describe it first-hand." Maybe not, but it's still there for anyone with uncensored Internet access to see.