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.