North Korea, led by inveterate saber-rattler Kim Jong Un, is drawing attention for a recent spate of missile tests. Last Wednesday, the isolated nation launched a medium-range missile into the Sea of Japan. The test signaled several things: That North Korea wants to be taken seriously as a regional power, it has weapons capable of hitting distant targets, and—perhaps most seriously—that it is developing stronger nuclear weapons.

Nuclear experts believe North Korea is prepping for its sixth nuclear test. And the signs aren't just out in the water. From satellite imagery, and other sources, they are looking for evidence that the isolated nation is planning to load a bomb onto a railcar, drive it deep into a mountain tunnel, and detonate—perhaps as soon as April 15, which is the birthday of the nation’s founder, Kim Jung Un's grandfather Kim Il-Sung.

That potential test, and all the very real incidents preceding it, already stirred up the geopolitical hornet's nest. Soon after last Wednesday's launch, President Trump told Japan's Prime Minister Shinzo Abe that "All options are on the table," in dealing with North Korea. Before meeting with China's General Secretary Xi Jinping this week, Trump said he would unilaterally confront North Korea if China did not rein in its unruly neighbor.

Whether or not this leads to a full blown crisis, the experts are keeping watch for signals of the test itself. “With any nuclear weapons program, you need to validate your engineering and your computer modeling,” said Joseph Bermudez, Jr., senior analyst at 38North.org, a project sponsored by the US-Korea Institute at the School of Advanced International Studies at Johns Hopkins University. He says this is North Korea's probable mode at the moment.

To verify his hunches, Bermudez has been poring over commercial satellite images taken every few days of North Korea’s Yongbyon Nuclear Complex, where the scientists make the bomb, and at the Punggye-ri Test Facility where the bombs are detonated inside horizontal tunnels that cut through the heart of a mountain. Bermudez compares images over time, looking for frame to frame changes. Is that water flowing down the mountain site drainage from bombsite tunnels, or just melting snow? Do piles of earth and rock mean the North Koreans are cleaning up an old tunnel, or building a new one?

It's guesswork buttressed by years of expertise. “We cannot look into the tunnel,” he said. “We cannot say definitively from satellite imagery alone that a test is imminent. We can say they have taken the steps to be able to conduct a test at any time of their choosing.” But he can say that this current activity is very similar to what he saw prior to nuclear tests in 2016. In the past two weeks, for example, he’s seen a large formation of people in front of the test site’s administration building, as if a high dignitary had come to the site. At the same time, the North Koreans have covered up equipment and supplies, and moved stuff around when commercial satellites don’t have an optimal angle during daytime fly-bys. There was also the arrival recently of an unusual airplane at a nearby commercial airport that signaled the visit of a North Korean VIP.

North Korea's goal with these tests is probably to boost their nuclear device's yield, while also making it smaller and lighter. One way to do that is to use less nuclear material—either plutonium or enriched uranium—and add deuterium or tritium gas, explained David Wright, co-director of the global security program at the Union of Concerned Scientists. These elements can also help super-charge the nuclear yield. “By pumping extra neutrons into the system, you can increase the amount of plutonium that fissions before it blows itself apart,” he said. Then they need to test it, to make sure the bomb itself can withstand the pressures of being launched atop a missile.

Once they settle on the nuclear recipe, the scientists will connect it to sensors to tell them what happened in the milliseconds between detonation and the time the sensors are fried to a crisp. “It’s as much a science experiment as anything else,” says Jeffrey Lewis, director of the East Asia Nonproliferation Program at the Middlebury Institute for International Studies in Monterey, CA. “You want all the basic data, so you have to install instrumentation, the cabling and sensors, even though it doesn’t survive the blast.”

And even though the tests are science, the pursuit is political. Whatever happens beneath that mountain has the power to shake the world.