Dane created the 'Disaster'. He caused a factory handling nuclear materials to blow up, and faked explosions around the rest of the city to push everyone Underground for their own safety. This allowed Dane Lawrence to keep his 'job' as Director of Principality. However, this decision wasn't made on a whim, Dane Lawrence had been planning the creation of the Underground since he found out his sacking from Principium was in the pipeline.
So Dane got to work. He ordered an excessive amount of lead plates. He placed these in store cupboards around the Underground. When people were in a frantic hurry, running away from 'nuclear war', it was Dane who came up with the idea of using lead plates to block the radiation out. In reality, he was locking people in.
After ordering the plates, he ordered medical supplies. He claimed he was preparing for an upgrade of the Principality first aid kits. But he had ordered enough to start a small hospital. He had ordered enough to start a Medicus. When people started falling ill Underground, a couple well placed medical packages made it seem like survival was possible.
This would not do though, what if people tried to communicate with the outside world? No, Dane would reinforce the tunnels. Logistically, this was just a construction project Dane accelerated along the timeline, but now every Underground tunnel had layers and layers more plating. If Dane had snuck a few lead plates there to block radio waves as well… it was just adding to the fortification.
Would people become nostalgic, would they miss being able to look up any information at any point? Dane made the archives. He began moving more and more books to the “share a book” section in the Underground. Eventually, just about anything you'd be curious about would be in one of the mini libraries around the Underground. Dane lead the effort to congregate them all in one station.
And finally, after all this planning and preparation, Dane was left with one thought to battle. How do you handle the unexpected? You try and be as prepared as possible. So Dane moved everything he could get his hands on to the Underground. He transported an olive branch, a sheep skull, a cello and more. Dane was prepared for any eventuality. The Underground was ready. A hidden gun in his pocket the whole time, Dane created the Underground.