According to some researchers, North Korea appeared to have conducted a secret nuclear test in 2010 and the explosion went unnoticed by the outside world.
Citing seismic analysis published last month, Michael Schoeppner of Princeton University and Ulrich Kuhn of the University of Hamburg say that, in May 2010, multiple monitoring stations in East Asia started picking up traces of unusual radioactive isotopes.
These traces of radionuclides that have been deducted are believed to have been the result of a nuclear test.
It wasn't the first time that the North Korea has been suspected of conducting a nuclear test.
In 2012, Lars-Erik De Geer, an atmospheric scientist with the Swedish Defense Research Agency, claimed in a military science journal that the North led two nuclear tests in April and May of 2010, citing analysis of atmospheric samples collected from over the Korean Peninsula.
So far these claims have not been officially confirmed.