As a former General Contractor, I'd like to say a few words about Underground Bunkers. Building underground is similar to, but on a much higher scale than building a house basement foundation. Both require proper drainage and waterproofing. Underground requires waterproof membranes, sealant, and I'd suggest insulation. Temperatures over 10 feet in the ground, in most areas of the United States, remain about 55 F most of the year, but you'd be uncomfortable at those temps for long periods of time.
I used "Underground Wood Foundation Systems" in my solar buildings. They were sealed and insulated with 2" Styrofoam, then covered with 6 mill poly before back filled with 24" of gravel, then excavated material, drainage pipe went around the perimeter. These buildings were all constructed in the Rochester, NY area...real winter!
The show "Doomsday Bunkers" appears to throw the steel building into the hole and cover it with back-fill...no insulation or drainage that I've seen. I thinks that's trouble down the road!