MR_rusty made a compilation of some of the most spectacular underground nuclear bomb tests. They occurred in the fifties in Nevada, New Mexico and Alaska. As he explained, some were small munitions, but others were huge nukes.
When the bomb is buried at sufficient depth, the explosion may be contained, with no release of radioactive materials to the atmosphere. However, the extreme heat and pressure of an underground nuclear explosion causes changes in the surrounding rock.
The rock closest to the location of the test is vaporised, forming a cavity. Further away, there are zones of crushed, cracked, and irreversibly strained rock.
Following the explosion, the rock above the cavity may collapse, forming a rubble chimney. If this chimney reaches the surface, a bowl-shaped subsidence crater may form.