A large truck was observed on the access road between the Guard Barracks and Southern Support Area on May 15, 2018, at the Punggye-ri site in North Korea.

Negotiations over denuclearization of North Korea collapsed this morning after North Korean dictator Kim Jong Un insisted the United States lift all economic sanctions in return for any nuclear disarmament.

U.S. Secretary of State Mike Pompeo said that talks with North Korea will soon resume, according to the Associated Press. However, before the Trump administration announced the lack of agreement, U.S. negotiators had already backed off the demand that Kim and his government allow access and transparency to the international community concerning their nuclear weapons program.

North Korea, like all countries with a nuclear program, is quite secretive about its research and testing. No one knows exactly how much nuclear material North Korea has or even exactly what kinds of warheads they've developed. [North Korea: A Hermit Country from Above (Photos)]

But North Korea won't necessarily have to let the entire world poke around its nuclear facilities to show that they've slowed or stopped their pursuit of nuclear arms. According to nuclear security experts, there are many ways to monitor the situation remotely — but they can provide only limited information without North Korea's cooperation.

"There is a whole panoply of technologies," said Sharon Squassoni, a professor and nuclear security expert at The George Washington University.

Testing, testing

North Korea has been claiming to be on the verge of shutting down its nuclear weapons program for as long as the country has admitted to having nuclear weapons. In 2005, then-leader Kim Jong Il admitted the country had nukes, and then signed an international statement promising to abandon its nuclear weapons program. In 2006, the country tested its first nuclear bomb.

That history of failed negotiations has security experts cautious about any potential for progress to be made between Trump and Kim, particularly since neither side has been very clear on what they consider "denuclearization," Squassoni said. Still, the meeting did represent an opportunity to bring North Korea back into a dialogue, said Alexander Glaser, the director of the Nuclear Futures lab at Princeton University. Even if North Korea refuses to share full information about its program, Glaser said, it might be possible to create a phased approach involving some remote monitoring and some onsite inspections that could prove whether the country is really meeting its promises.

The easiest aspect of the program to track is whether North Korea is actively testing nuclear bombs. North Korea's cooperation is not required. Nuclear explosions are pretty obvious, and the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) already runs a commission to monitor the atmosphere, oceans and subsurface for any testing. Infrasound monitors are capable of detecting aboveground explosions, and underwater microphones can detect undersea testing (both of which were banned under the Partial Nuclear Test Ban Treaty of 1963).

Underground nuclear tests show up on seismometers that are designed to detect earthquakes. There are many such arrays, run by research organizations, governments and even private entities, and quite a few of those upload all their data online, said Jeffrey Park, a geophysicist at Yale University. That means that anyone with an internet connection can detect an underground nuclear test, as long as they know what to look for. [The 22 Weirdest Military Weapons]

"We ordinarily have fairly good ideas about where nuclear testing is going on," Park said, "So any kind of tremor near a nuclear test site attracts a lot of attention."

Nuclear tests create a lot of what geophysicists call "p-waves," which are compressional waves created by the big blast pushing everything outward, all at once. These waves look quite different from the signals created by earthquakes, Park said. Earthquakes are caused by faults sliding side-by-side, so their seismic signals are dominated by shear-wave energy.

Knowns and unknowns

Thanks to remote seismic monitoring, the international community can tell within seconds to minutes if Kim's regime has denoted something at its underground testing site, Punggye-ri. By triangulating the source of waves detected at different seismic stations, scientists can even tell exactly where at the site the explosions occurred, even if they were as close as a kilometer apart from one another. North Korea detonated bombs at Punggye-ri in 2006, 2009, 2013, 2016 and 2017. The first two tests are widely considered to be failures, Park said. The 2013 and 2016 tests, he said, were indicative of a first-generation plutonium fission bomb, not unlike the bomb dropped on Nagasaki in 1945.

North Korea claims that the 2016 and 2017 bombs were both thermonuclear, or hydrogen bombs, which generate explosions via nuclear fusion rather than fission. Some outside experts think the North Korean government really does have a thermonuclear bomb, though others, including Park, are skeptical. For the purpose of gaining recognition on the world stage, Pyongyang would like everyone to believe its nuclear program is strong, Park said, but it's not clear that the testing done so far indicates the existence of a thermonuclear bomb.

"There's a lot we don't know," Squassoni said.

Many of those unknowns are challenging to fill in without cooperation from Kim's regime. For example, Squassoni said, North Korea has only one plutonium reactor, so outside experts could make an educated guess as to how much plutonium the country had to work with. But intelligence operations and one 2010 tour given to Stanford University experts have revealed that North Korea can also enrich uranium, which is done in facilities that are far easier to hide than a huge reactor. There is at least one uranium-enrichment facility in the country, Glaser said, and probably at least one more at an unknown location. (Either uranium or plutonium can be used to make nuclear weapons.)

"There may even be a third site that we are not aware of," he said.

Another easy-to-conceal facet of the nuclear program is the development of delivery systems. It does North Korea little good to have a 1945-style bomb, Park said; those require delivery by enormous bombers. What the country needs to be truly threatening is a warhead that can be delivered by missile. North Korea suspended missile launches in 2018, and maintaining that moratorium was almost certainly part of the negotiations in Hanoi, Glaser said.

Remote cooperation

Learning about what's going on inside nuclear facilities is a tough challenge, said Squassoni, who once worked in the U.S. State Department and who is now on the board of the Bulletin of the Atomic Scientists (the group responsible for the Doomsday Clock). Informants on the inside are hard to come by. And North Korea is not likely to hand over a list off all their facilities to the international community. [Doomsday: 9 Real Ways Earth Could End]

"We have a ballpark sense of the nuclear program, but I'm sure there would be some surprises if we got access," Squassoni said.

If the North Korean government were willing to let out even a little information at a time, the world could monitor much of their activity from afar, Glaser said. Satellite reconnaissance can be used to ensure that there is no activity at plutonium- or uranium- production facilities; the same can be true for missile-launch sites (which are still being maintained despite the moratorium on launches). Air monitoring and soil or vegetation samples could show any hint of production of radioactive materials. With enough information and enough time, scientists could conduct a sort of "nuclear archaeology," Glaser said, by figuring out how much uranium had been mined in North Korea and then comparing that to the number of warheads the country claims. That accounting could make it clear whether the country was hiding anything.

Even in a best-case scenario, confirmation of denuclearization couldn't happen overnight, Glaser said.

"It will take years to confirm the completeness of the declaration, or to have high confidence in the absence of undeclared items," he said. "There is no way around this."

Originally published on Live Science.