Reinforcing dirt walls is imperative with an underground shelter.
Underground shelter blueprints.
Adapt or Die
Atlas Survival Shelters-Underground Shelters-Bomb Shelters
This is an air exchange unit for underground shelter's.
Z Day - solutions to the problem of the undead
How to Build Underground Shelters With Cinder Block Construction | eHow
Habitat"s for the Heartland
My ideal Shelter
Underground Shelter, Sebastian Kowoll on ArtStation at http://www.artstation.com/artwork/underground-shelter
Inhuman Earth: Urban Environments
Survival Center - Underground Shelters, Bomb Shelters
Vivos 2012 Underground Shelter