If you have looked into bunkers you may have read a lot about using shipping containers to do build them out of. I need you to read this article why you shouldn’t bury shipping containers before you read about box culverts. It will make more sense if you do.
Box Culverts are concrete tunnels they put under roads. They are designed to be buried and are very customizable. There are companies that can make these culverts to size and your specifications.
You pay out of your nose for a reinforced shipping container that you hope doesn’t get rusty and leak over time. For a typical reinforced 10ft shipping container you could probably pay about 10k. For the same sized customized concrete culverts you are looking around 6 – 8k.
This leaves you around 2 – 3k dollars to spend on better air filtration or food and water to stockpile in your shelter. Concrete will withstand blasts better than an old shipping container too. People always tend to miss that point! Check out the thread I found about these culverts and be sure to read all the comments. A lot of great information there.