Yesterday saw the first nuclear test since 2009 and North Korea's largest detonation to date, over six kilotons of force generated from an underground site 20 miles west of the town of Hoemun. It's a powerful reminder that the country is still nuclear, and still dangerous. But for all the diplomatic damage they've incurred, it's hard to say what they've gotten back. In fact, it can be hard to say why the test happened at all. For a country that's already known to have nuclear capability, what good is one more test?

There are plenty of testing methods that don't require full-scale detonation

First, the downside: it's dangerous and, unlike the rest of the nuclear process, extremely hard to keep secret. Countries often keep development as covert as possible, often pulling together materials through espionage and unsanctioned trade. The test makes everything public, and dramatically so. In North Korea's case, we already know the rough size of the blast and scientists are searching for airborne particles that might mark it as a uranium or a plutonium bomb. These details carry great diplomatic and political weight, and Kim Jong-Un would certainly prefer to keep at least some of them secret — but after the test, that's simply not an option.

There are ways around this. Any industrial product needs to be tested before you can be sure it works, but there are plenty of testing methods that don't require full-scale detonation. Countries can test the individual components of the bomb in a lab, or set off a version of the bomb with a fraction of the usual fuel: enough to give off some portion of nuclear energy, but not enough to achieve the critical mass required for an explosion. These sub-critical tests can verify the nuclear reaction without the literal and metaphorical shock waves of an actual blast.

More than two thousand nuclear tests have been staged in the past 70 years

But that hasn't stopped anyone from setting off nuclear bombs. More than two thousand test explosions were set off between the beginning of the nuclear age with the United States’ Manhattan Project in 1944 and the more recent Nuclear Test Ban Treaty in 1992. The US was responsible for more than half of the two thousand detonations. Initially, there were valid hypotheses to test: what happens to a house that's 3,000 yards away? What are the effects when a bomb is detonated at the edge of the atmosphere? How about underwater? Researchers tested out a lot of projects that were never implemented, including the prospect of disrupting a naval fleet with a submerged detonation. The only way to know for sure what worked was to try it out.

The purpose of Tuesday's test was more political than technological

But by the 80s, the most pressing questions had been answered and the US was still detonating bombs nearly every month in Nevada, Mississippi, and the Marshall Islands. Blasts were confined to remote islands, deserts and (most often) sealed mineshafts, so the radiation could be kept away from civilians. Increasingly, the audience for the tests wasn't scientists but the Kremlin, keeping tabs from thousands of miles away. All the US wanted to prove was that the bombs still worked, and the Pentagon could use them any time it wanted. The Test Ban Treaty came only once the Cold War had ended, and the world’s superpowers had nothing left to prove.

The purpose of Tuesday's test was similar, more political than technological. Even if North Korea has a few lingering questions about the effects of its bombs, there are more discreet ways of getting information than full-scale detonation. This was more about sending a message and flexing its nuclear muscle — "a declaration of [North Korea’s] technical proficiency and military power," as The Economist called it this morning. It's still unclear what North Korea's missile capability is, and whether they'd be able to launch missiles far enough to reach American (or even Japanese) soil, but politically it may not matter. As long as they can move the seismograph needle, they'll have the world's attention.