South Korea accidentally discovered a bunch of tunnels that the North had been prepping. They'd gone unnoticed until performing some sort of water investigation iirc. You used to be able to go into them (near the DMZ) on tours. The walls are black and the guide I had noted that upon discovery, the fleeing workers painted the wall with coal to "make it seem like it was a mining operation all along".
So, the "drink your milkshake" level access isn't crazy.
No body/tank is getting through the ironically named Demilitarized Zone dividing South and North Korea, without getting lit up with fast moving pieces of metal hitting with high kinetic energy.
Hence the tunnels.
North Korean government's long standing, highest priority goal is conquest of South Korea through any means, including military invasion.
So the epicenter ends up in the attacking country’s border (or at least within a few miles)? Doesn’t sound like a particularly useful weapon when you’ve got to attack your own country with it too.
that was a plot in one of the paperbacks in 198x - back then there was a spike in detection of USSR submarines violating Sweden waters, and in the story the submarines were performing under water drilling and injecting some stuff to be able to trigger earthquakes on demand when the time comes.
