As tensions between the United States and North Korea mount, there is a group of scientists watching the rogue country around the clock.
When an earthquake happens a seismologist, with the help of instruments, can usually say how large it was and pinpoint the epicenter. But they do much more than that.
According to the Seismological Society of America, most of what we know about North Korea's past nuclear tests comes directly from work by seismologists.
These scientists study earthquakes and energy waves moving through the ground.
When a nuclear blast occurs, energy waves are created and studying them can pinpoint where the explosion happened.
On September 10, 1996, the U.N. General Assembly adopted the Comprehensive Nuclear Test Ban Treaty, which banned nuclear explosions anywhere on earth. It was intended to prevent nuclear testing worldwide in the atmosphere, underwater, and even underground.
"Our signature along with Russia, China, France, United Kingdom and the vast majority of nations around the world will create an international barrier against nuclear testing," former President Bill Clinton said at the time.
However, North Korea's rogue regime is the only country to ignore the rules.
"Seismology evidence is key to alert one's leaders and the international community on anything that happen on the planet," said Dr. Lassina Zerbo, who is the executive secretary of the Comprehensive Nuclear-Test-Ban Treaty organization, located in Vienna, Austria.
The organization has more than 300 monitoring sites around the world.
Dr. Zerbo said, "The Comprehensive Nuclear-Test-Ban Treaty organization is an independent international frame work that gathers 183 countries that have signed and 166 that have ratified. So it is true, North Korea has yet to sign the comprehensive test ban agreement."
He noted that it was his organization that discovered and relayed information about North Korea testing nukes in 2006, 2009, and 2013. That data was sent to all 183 countries who are a part of the organization within a few hours -- faster than North Korea could go public about its own test.
Dr. Zerbo said the organization never tells a country what policies to create after, for instance, scientists discover a nuclear test was performed. He says they simply give world leaders the data and let them reach their own conclusions.