Radionuclide station RN16 detected North Korea's first test.CTBTO A global network of sensors designed to verify nuclear testing has failed to pick up radioactive gases from North Korea's nuclear blast. The lack of gases, which are created in large amounts during a normal nuclear detonation, has puzzled non-proliferation experts and scientists around the world. But few doubt the nuclear nature of the test, which the US government reported as "a few kilotons" in size on Monday.North Korea conducted what it claims was its second nuclear test on 25 May (see 'North Korea's bigger blast'). Within seconds, a global network of...