/diy/ - Do It Yourself

Here's a small diagram to help explain.
essentially the further underground you go the more problems you get and the less benefit. Dirt isn't magic, it's far weaker then cement, you might as well have a thicker structure above ground in almost all cases.
A structure build below ground has a tendency to get buried, the access points to it get blocked and it becomes a tomb.
A structure fully above ground can be battered but even if it's only brick that will survive even a nuclear blast at a reasonable radius, it will take an enormous amount of radiant heat but this can be mitigated further down the line even by things like reflective paint or air vents.

The structure that's a little underground, a little mounded and a little buried is the happy middle ground.
Only 30% exposed and it's right near the top where the roof corner reinforces it, it takes minimal heat, blasts are deflected over it, you can get inside it really quickly, debris piling up on the sides doesn't tend to trap you and in fact can build a protective layer in the case of a cyclone or the like.