Last weekend, North Korea responded to a UN Security Council condemnation of its missile launches by warning of “next-stage steps which the enemy can hardly imagine,” including “a new form of nuclear test for bolstering up its nuclear deterrence.”

Golly, that sounds awfully hostile.

A “new form” of nuclear test? My thoughts immediately turned toward Pyongyang’s next step in its nuclear weapons development. Pyongyang might test a device using highly enriched uranium, if they haven’t done that already, or start down the path toward tactical nuclear weapons or perhaps burning thermonuclear fuel. It is hard to know Pyongyang’s near- and long-term technical goals for its nuclear arsenal, although I suspect they have such goals.

The original Korean, however, suggests that something new about how North Korea tests, not what it tests. After checking with a number of Korean speakers, the Korean phrase—“핵억제력을 더욱 강화하기 위한 새로운 형태의 핵시험도 배제되지 않을것”—appears to refer to a new form of testing, as opposed to simply a new device.

North Korea says we can hardly imagine what that might be, but lets try anyway, shall we?

The simplest explanation is that North Korea may conduct simultaneous detonations of two or more nuclear devices. Most nuclear powers conduct simultaneous detonations, or “salvo tests,” in order to conduct more tests in less time. None of the test personnel live year round at the nuclear sites, which are often located in remote areas. These areas are often remote because the weather stinks. So the idea of getting two-for-one (or even three-, four- or five-for-one) while the scientists are on site has plenty of appeal, even if it makes the tests more complex. Here is how the Russians explained it:

For underground nuclear explosions the special technology of salvo nuclear tests was developed, when two or more nuclear devices were simultaneously detonated within one nuclear test. This technology represented an essential step forward in comparison with testing of individual nuclear devices because it allowed [the Soviet Union] to intensify testing activities, even if its realization required some increase in the complexity of the experiments. This approach was advanced both in the USSR, and in the U.S., but was more widely practiced in the USSR, apparently due to the more severe weather conditions at the nuclear sites and lower financial and material capabilities. The Soviet Union conducted 146 salvo nuclear tests in which 400 nuclear devices were detonated, while the United States conducted 63 salvo nuclear tests in which 158 nuclear devices were detonated.

Those same factors—lower financial and material capabilities and severe weather—are also present in North Korea. In particular, the North Korean test site has lousy winters, followed by spring floods. This would fit well with my hypothesis that North Korea’s tunneling at Pyunggye is intended to support more intense nuclear testing than North Korea has conducted to date. On Friday, North Korea hinted at this, warning that the United States should expect “more annual and regular” efforts to bolster and demonstrate the DPRK’s “war deterrent.” That general statement suggests we should expect more missile launches and nuclear tests in the coming years.

There are other, more speculative possibilities. North Korea has conducted its previous tests in tunnels drilled horizontally into the mountains around the test site. The size of bombs that the North Koreans can test in these tunnels is limited by the size of the mountain and the resulting overburden. (How deep does a nuclear explosion need to be buried? The standard equation is Depth in meters = 120 × (Yield in Kilotons)1/3.) A back of the envelope calculation suggests a few tens of kilotons is the most that the mountains at the current test site can accommodate.

Larger tests, then, would need to be conducted in shafts drilled vertically much, much deeper into the ground. Shaft tests are usually limited to several hundred kilotons—although you can always dig deeper. (The United States conducted a 5 megaton test in Alaska, Cannikin, in a 1,860 meter shaft! No, I don’t think Sarah Palin could feel that from her house. Why do you ask?) At some point, though, drilling deeply is harder than simply scaling the test, given the costs of drilling and challenges of testing below the water table. This would probably require a different test site than Punggye-ri. So far, there are no reliable reports of a second test site, but who knows.

Of course, one needs to be careful or underground tests may release radioactivity into the atmosphere. This happened to the United States with the Baneberry test, which much to the chagrin of its team, vented radioactivity. Oops. The author of one US report on containing underground nuclear explosions offered this wry observation about the event:

(In case you want to read more about underground nuclear testing, I recommend: Office of Technology Assessment, The Containment of Underground Nuclear Explosions, October 1989, and James Carothers et al, Caging the Dragon: The Containment of Underground Nuclear Explosions, Department of Energy/Defense Nuclear Agency, 1997.)

This raises the last possibility that North Korea might consider atmospheric nuclear testing—something that is prohibited by the Limited Test Ban and the Comprehensive Nuclear Test Ban Treaties, neither of which, North Korea has signed. North Korea has taken special care to point out that the nuclear tests have no negative environmental impact. That may well be because they are reluctant to inflame Chinese public opinion. The Chinese government released radiation readings after the last nuclear test, to reassure the public in Northern China.

Atmospheric testing is tricky though—I just completed a draft of a book that contains a chapter on China’s nuclear testing program, which is largely a story of managing international opposition to atmospheric nuclear tests while trying to master the technology of testing underground. (This is, in fact, the origin of China’s no-first use policy. But more on that later.) North Korea has the same constraints, only in Pyongyang’s case, the downwinders are Pyongyang’s primary lifeline.

I suspect, for the moment, that Chinese public opinion is enough to keep North Korea’s nuclear tests underground.

There are, of course, things that might change. If North Korea wants to develop thermonuclear weapons—something the Chinese would achieve with their fifth and sixth nuclear tests—North Korea might want to test these very large devices at full yield, which probably means atmospherically. And then there is the precedent set by China’s fourth nuclear test—a live warhead delivered by a live missile. The United States had denigrated China’s nuclear program then, as it does North Korea’s now, by claiming that a few tests did not mean that either party was capable of actually arming a missile with a nuclear weapon. The Chinese wanted to leave no doubt about that, firing a nuclear-armed missile from a missile test center to the Lop Nor test site. North Korea might conduct atmospheric testing if Kim Jong Un feels the need to develop thermonuclear weapons or perhaps, like China in 1966, wants to demonstrate the ability to deploy nuclear-armed missiles or artillery. A nuclear-armed Musudan would get my attention. He’s only likely to do that, however, if he no longer cares what Beijing thinks.

For now, I am inclined to believe that “new form” of nuclear testing most likely means simultaneous tests, part of a program of more intense nuclear testing that we are likely to see over the next few years. Still, it is useful to remember that Kim Jong Un has a number of other unpleasant provocations from which he might choose.

Jeffrey Lewis is Director of the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies (CNS), Monterey Institute of International Studies, and a frequent contributor to 38 North.