None of the components of a global network of sensors, meant to pick up traces of radioactivity that are regularly generated by nuclear tests, managed to accurately identify gas signatures coming from North Korea's test, in late May. Thus, the Western World has no conclusive evidence that Pyongyang actually detonated such a device, other than tremor records, which were picked up by seismographs in Japan.

On Monday, the United States reported that the power of the device detonated in North Korea, which American officials have no doubt was nuclear, is in the several kiloton range. However, this claim does not match field readings, which show that none of the gases regularly generated in large amounts after such a test could be identified in the area around North Korea, or elsewhere in the world. Scientists and non-proliferation activists are puzzled by this mystery, which is fairly difficult to explain.

Seismographs belonging to the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), an international group that will enforce the global ban on nuclear testing as soon as enough countries ratify the treaty, have picked up the 4.5-magnitude earthquakes generated by the North Korean test on May 25th. Within seconds, the global network of sensors showed that the blast, the country's second test, was much larger than the first one, done in 2006.

The CTBTO inferred that the signature of the tremors hinted at a man-produced event, possibly of nuclear origin, but its researchers had hopes that a follow-up set of measurements would confirm these suspicions, Nature News reports. However, the readings failed to show that up even now, about a month after the blast. By now, they should have been picked up several times over, military experts and analysts say.

The main gas scientists were hoping to identify is made up of xenon isotopes. This noble gas has the ability to filter through rocks, after underground testing, for instance, and, once in the atmosphere, it can spread for thousands of miles around the original blast zone. The gas does not react with anything in the atmosphere, and it can be accurately identified weeks later. For example, back when NK did its first nuclear test, in 2006, a station in Canada picked up the signature, some two weeks later.

Some analysts believe that North Korea may have invested heavily in sealing its test tunnels, so as not to allow for the gas to get out. That is to say, a strong enough blast would ensure that the rocks around the test site liquefy, creating an air-tight chamber that would not allow for the gas to be released into the atmosphere. This would explain why the previous test released the xenon mix, as the power of the bomb was too small to cause such a reaction in the rocks around it.