North Korea in final preparations for nuclear test: report
The images, taken March 25, show up to four vehicles or equipment trailers continuously present at the entrance to the Punggye-ri Nuclear Test Site, with communications cables likely laid on the ground.
"This equipment would likely be used to initiate the test, collect data from the explosion and process the data," said 38 North, a project of the US-Korea Institute at Johns Hopkins University.
The researchers noted that water was being pumped out of the portal and drained downhill to keep the tunnel dry for monitoring or communications equipment.
"The combination of these factors strongly suggests that test preparations are well under way, including the installation of instrumentation," the researchers said.