The DPRK's fifth and largest nuclear test took place inside the country's Punggye-ri nuclear facility. The test site is top secret, and little is known about what goes on inside.

Punggye-ri Nuclear Test Site is the DPRK's only known nuclear test site. It is situated in Kilju County of North Hamgyong Province. Pyongyang began nuclear testing there in 2006.

The facility is located near Mantapsan mountain. The terrain makes it difficult to observe testing at the site and keeps radio-active material from leaking into surrounding areas. Ideal conditions for nuclear testing. 

According to South Korean media reports, DPRK's first nuclear test took place inside the facility's east tunnel, while the second and the third tests took place inside the west tunnel. The west tunnel reportedly has a spine-like structure -- with around a dozen iron doors. The tunnel's design effectively blocks radio active material from leaking outside during and after nuclear testing.

Experts say the DPRK conducted its fourth nuclear test inside an extension line connected to the west tunnel earlier this year. The latest nuclear test reportedly took place 9 kilometers west of the site of the fourth nuclear explosion.