Maybe they sent in a swarm of robot flies to go down into the tunnels and take pictures.
The Chinese and Koreans are probably reading this as I type it, so we might be phishing, hearing there are tunnels and after the release, listening.
This would make much more sense than sending men in. It also might make sense to see the reaction to this information. As someone points out...see where they place their assets and what they want to protect from prying eyes....