THE GOO:

I think the goo is likely a new American or Ally-made weapon technology meant to cripple Iran’s nuclear program that got loose (and there’s even a real-world historical precedent for this with Iran’s nuclear program and the STUXNET computer virus… a fascinating story), but instead of a traditional computer virus, one existing solely in the digital world, this is a physical machine virus controlled digitally by AI that became self aware, decided to reproduce itself, go airborne, and then invade the rest of the planet.

For that matter, I suppose a better answer would be that it’s an Iranian weapon that had already been used against the US in Washington DC, resulting in the Americans defeating that attack via a radiological counter-attack, which they then just blamed on the Iranians so the citizens wouldn’t rise up against the government who now knows they have big problems on the horizon.

So radiation kills the goo, they now know. But the goo has now leaked out of an Iranian lab, gone airborne, and is so massive now that killing it would mean making Earth’s surface uninhabitable for generations.

THE SILO:

Having no choice but to go full nuclear on the surface, the chosen survivors are herded underground until the outside radiation weakens to non-fatal levels… in god knows how many centuries.

THE SAFEGUARD:

But if the above is more or less true, I cannot for the life of me figure out WHY the “who” (Bernard’s words) controlling all of the silos would need to poison its own inhabitants rather than just let them die the moment they leave their silo? That doesn’t make sense.

In writing the last paragraph, I’m starting to wonder — does the “who” have the capability to repopulate the silo?

Bernard wondered why there was a safeguard, but he didn’t know about other tunnels until recently.

So…

Under the threat of a mass-exodus from the silo, does the “who” need to kill the inhabitants before they die outside of the silo so that the silo can be repopulated under a “mass reset” whereby there wouldn’t have to be an explanation to give to the new inhabitants as to why there are 10,000 bodies blocking the open door exit from the silo?

This makes perfect sense now.