Reflective of the paranoia of the fallout shelter obsessed 1950s, The Unknown World (1951) demonstrates the fear and extreme thoughts entertained in the hypothetical scenario.
A group of scientist have come up with a plan for the human race to survive the inevitable Atomic War.
They come up with a plan to design a machine (a cyclotram) that will allow them to burrow towards the center of the Earth, live and thrive anew in the caverns, fissures and air pockets that pepper the planet's crust.
The initial plan falls through, due to financial reasons. Eventually, a wealthy benefactor finances the expedition on the stipulation that he be allowed to come along on the maiden voyage. The machine is built and they enter the Earth through an extinct volcano.
Inside the planet, they find various caves and caverns, but nothing that seems to meet their needs.
They travel further, a thousand miles or so, and the unexplored terrain proves treacherous. They lose a few members of the crew in the process.
Meanwhile the rich patron is falling for the sole female scientist, and she seems receptive to his advances.
Further on, deeper into the Earth, they find a promising spot, but the realities of living in such a place seem all the more unreasonable. They must make tough decisions and tensions are growing.
Do they stay, abandoning all the glory of the Earth's surface... or not?
clip from The Unknown World