North Korea has conducted a slew of underwater-launched ballistic missile tests since 2016, but all launches were made from the same 2,000-tonne-class submarine which has a single launch tube. Many experts call it a test platform, rather than an operational submarine in active service.

The group entered the country through China on their trip to Rason in the far northeast — more than 500 miles from the capital city of Pyongyang, which once occasionally welcomed occasional tourism groups, albeit under strict supervision and no real freedom of movement.