What did Lucas tell Bernard at the end?

No book spoilers please.

Just finished the show and a little confused and feel like I missed some details.

1. Why were the silos built? Was it revealed that a nuclear bomb went off and that’s why they are in the silo?

2. What did Lucas tell Bernard? What did Lucas see? I kinda missed what happened during that scene of Lucas but was there a door that Lucas went in and discovered something?

3. The voice at the end says that they will activate the safeguard procedure if Lucas says something. What could be so bad that they will kill or so something to everyone?

4. Is it really poisonous outside? Solo said that there was a way that people made it safe to go outside. So why would they keep the people in the silos?

Please please please no book spoilers. And if something was kept a secret in the show for a reason, don’t mention theories.

I love this show so much though!