Dugout

A dugout is a type of dwelling, shelter, or other structure that is created by digging a hole or depression into the ground. If this is meant to be a more permanent structure a slight above-ground structure with a roof, or basement like doors can be built above it to allow for consistent re-entry.

Back in the day dugouts were often carved into the sides of hills, embankments, or ravines and were protected by a crudely constructed front wall built with logs, leaved branches, or sod. Reverting back to this primitive shelter may be your best option when having to bug-out in a hurry.