The ancients built these underground tunnels to protect themselves from what was about to happen above ground. As said, maybe aliens had come to give them the technology to accomplish this. Could it be protection from natural disasters or the Anunnaki?
Fast forward to today...large underground tunnels, bunkers, and cities are being built all over the world, including the seed vault in Norway. The big difference today is that they can't make an underground city large enough to hold everyone. This is why any actual doomsdsy event will be hidden from the general public who will resemble a deer in headlights when an event occurs.
Good post Snake...they mentioned "Snake People" in the vid...acquaintances Snake?