If a nuclear bomb can eviscerate through building/anything laterally, why does it not penetrate downward into the earth as it expands?

The reason is than detonation takes place at least 2,000 feet above the ground and generates a spherical shock wave whose effect on the ground is highly damped the the air cushion .

> Is the material of the ground that strong?

err, no. there just is so much of it. Radiation will penetrate several meters of ground, but every meter will reduce the amount that gets through, by a fixed factor. After a mile of ground, nothing remains

The destruction of a nuclear blast is maximized by detonating at altitudes of 2000 feet or more.

The earth is saved by this altitude. It does leave a crater but it is not extremely deep. the ones in japan were only about 10 feet deep. The buildings were not eviscerated(disemboweled) they were devastated(flattened) within the first mile of the blast. concrete melted but in WWII japan there wasn't much concrete, it was mostly paper and Bamboo which evaporated.

It does depending on how high it detonates and the magnitude of the blast...

N.Shadows

Underground nuclear explosions typically push the dirt and rock outwards, to make a cavity as much as 100 meters across. On the surface, the blast bounces off the ground, so I would not expect the hole to be more than 10 or 30 meters deep.

If you go laterally, very few buildings have more than 10 meters of solid steel/concrete/brick.

it does blast away the layer of surface dirt, the layer of surface dirt isn't that deep in most areas, then there is the bedrock, which is really hard and thick,