When the waves bring trash onto the beaches of front-line South Korean islands, Kang Dong Wan can often be found hunting for what he calls his “treasure”—rubbish from North Korea that provides a peek into a place that’s shut down to most outsiders. “This can be very important material because we can learn what products are manufactured in North Korea and what goods people use there,” said Kang, 48, a professor at South Korea’s Dong-A University.

He was forced to turn to the delicate information-gathering method because Covid-19 has made it much harder for outsiders to find out what’s going on inside North Korea, one of the world’s most cloistered nations even without pandemic-led border closures.