On January 6, 2016, officials from the Democratic People’s Republic of Korea announced that they had successfully tested their first hydrogen fusion bomb in an underground detonation, in North Hamgyong Province. North Korea’s media claimed that this new weapon would be used as a self-defense measure against a U.S.-backed invasion, but experts are skeptical that the DRPK’s new weapon actually exists.

Two types of data are considered to be the key to determining whether or not the DRPK’s claims of testing an H-bomb are true: seismological and chemical. Seismic waves, typically generated by natural geological movement, can also be created by sufficiently large explosions, such as nuclear detonations. However, the seismic data gathered from North Korea’s explosion only indicated that it was a small device, under 10 kilotonnes — roughly the same yield reported from the fission bomb test reported by Pyongyang in 2013, and only one-tenth the yield that even a modest fusion explosion should have produced.

The second data source — the chemical signature — is still being sought. A thermonuclear fusion detonation uses a small, old-fashioned fission bomb as an initiator to produce the temperature and pressure needed to start the fusion chain reaction in the bomb’s deuterium fuel, an isotope of heavy hydrogen. These detonations leave behind isotopes from both fuel sources, both from un-reacted elements, and from new elements created in the detonation.

Experts from the Vienna-based Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) are waiting on results from atmospheric monitoring efforts that are currently underway. They are watching for varying isotopes of xenon, a gas that can permeate the rock that contained the explosion: the ratios of the different isotopes of xenon can be used to determine if they came from a lower-yield fission bomb, or if they were generated from a fusion bomb, as Pyongyang officials claim.