Someone in last bread posted a screenshot from 4chan, and even though the person who wrote the comment doesn't think it's what is happening, it certainly seems plausible:

"You would think that they could use thermal or ground penetrating radar or even xray or something, satellites, to find the tunnels and the people down there. however, with the other billions of us on the surface, driving around, working and living, you can't discern what is on the terrain and what is subterranean. you would have to stop the whole world and tell them to go home in order to use machines to image the tunnels from outside. I honestly don't think that's what is happening here though"