Testing in the atmosphere and underwater was banned by the 1963 Partial Test Ban Treaty, after public concerns over radioactive fallout from tests. The U.S., the Soviet Union and Britain signed the treaty, but nuclear powers France and China did not. France conducted its last above ground nuclear test in 1974; China in 1980.

To develop the technology for its atomic weapons program, North Korea needs to conduct tests to perfect the ability to miniaturize a nuclear weapon so that it can be mounted on a missile, delivered without mishap and hit its intended target.

How will we know if North Korea carries out an underground test?

Once a nuclear device is exploded, seismographs around the world will detect the movement. "Manmade explosions have a very different signature than natural events," Thunborg said.

"Seismic technology is the core function of any monitoring system, and we are running the only international system that is dedicated to ensure that no nuclear explosion will go undetected in compliance with the comprehensive nuclear test ban treaty."

The CTBTO operates a global system of more than 300 monitors -- such as seismic, sonar or radiation sensors -- in 85 countries across the world. Even if no radiation escapes into the atmosphere, underground explosions leak noble gases into the atmosphere which CTBTO sensors can detect, Thunborg said.