Subsidence crater

A subsidence crater is a hole or depression left on the surface of an area which has had an underground (usually nuclear) explosion.

Subsidence craters are created as the roof of the cavity caused by the explosion collapses.

It is at the point that a chimney is formed through which radioactive fallout may reach the surface.

This type of mound has been called "retarc", "crater" spelled backwards.

This phenomenon is called "cratering" in oil field slang.

Post-shot subsidence crater and Huron King test chamber, which was less than 20 kilotons (1980)
Rubble mound ( retarc ) formed by the Whetstone Sulky explosion.