A Google Map view of North Korea's suspected nuclear test site, based on USGS data from the resulting earthquake, is indicated at the green arrow. The red button shows the suspected site of North Korea's nuclear facility. (Screenshot by Washington Post)

Probably the most pressing question still lingering over North Korea's latest nuclear test is what sort of nuclear material the country used: was it plutonium or highly enriched uranium? But it's also a surprisingly difficult question to answer, a reminder of the country's success in controlling what we do and don't know about what happens inside its borders.

The question of what sort of nuclear bomb North Korea detonated might sound like a boring technical detail, but it matters, and for more than just East Asia. North Korea has previously used only plutonium in its bombs, but if it's now detonating uranium, that would be bad news for four reasons:

1. North Korea would have two ways to build a bomb, which means a potentially larger arsenal.
2. The country has a natural supply of uranium and can enrich to bomb-making levels in secret; plutonium is limited and is much tougher to hide. So its weaponized uranium would be tougher to keep track of and easier to make in larger quantities.
3. Iran uses uranium in its nuclear program, so North Korea could share research and lessons from the nuclear test with Tehran.
4. Uranium is easier to ship abroad, meaning North Korea could more easily sell it.

The world is watching closely for clues as to what sort of material North Korea might have used. A representative of the United Nations' Comprehensive Nuclear-Test-Ban Treaty Organization, which monitors nuclear activity, told Fox News that officials were still trying to figure out whether it was a uranium or plutonium test.

There are a couple of possible clues, but at this point that's all they are. North Korean state media bragged that Pyongyang's nuclear deterrence is now "diversified," which might hint that it started using uranium. On the other hand, state media also claimed that North Korea had "miniaturized" its warhead, which would make it more suitable for use in a bomb or even a missile; Reuters reported that plutonium is the better-suited material for this purpose. Although when I asked Jeffrey Lewis, a nuclear proliferation expert who specializes in East Asia, whether or not this was accurate, he responded, "No, that seems to be speculation. Good news is the author has a 50/50 chance of being right."

So how do we find out for sure? That "requires quick detection and analysis of the different types of xenon gases produced in an atomic explosion," according to a recent New York Times story. The United States has special, highly sensitive airborne monitoring equipment and radiological stations around the world. The xenon gas has to be measured within the first 10 to 20 hours after the explosion, but the problem is that it can take a few days for the gas to leak out of North Korea's underground testing facility.

As an indication of just how tough the underground explosions are to measure, the world still doesn't know for sure what kind of material North Korea used in its last nuclear test, in 2009. Presumably, the bomb was plutonium: we know North Korea has a stockpile of plutonium, and its uranium-enrichment efforts weren't discovered until the next year. But no gas was reported to have leaked from the 2009 test, so we can't say for sure it wasn't uranium.

The mystery around North Korea's nuclear material ends only if the country slips up somehow, which it didn't in 2009 and hasn't so far this time around, or if Pyongyang comes out and tells us. It's a reminder of North Korea's stranglehold on information, the remarkable advantage it still maintains over the United States, despite its relative weakness and poverty, when it comes to controlling what we know about the events inside those dark borders.

Some analysts have speculated, since before Tuesday's detonation, that North Korea might be preparing for two separate tests: one uranium and one plutonium. If that happens, there will be hints beforehand, but we won't really know until the ground starts shaking.