Has nobody realised yet that these 'sinkholes' are the result of direct energy beam weapon testing?
Read Dr. Judy Wood on her 911 theory (even she hasn't twigged yet).
Ask yourself the question: why are they all perfectly round, regardless of the ground material? That's NOT a natural phenomenon.

then, probably alien space crafts are leaving Earth because something "nasty" is coming from space and it is becaming unsafe fot them :-)

Most sinkholes are caused when a cavity forms underneath the surface, either suddenly (water pipe breaks) or over a longer period of time, and its "roof" finally collapses.

I think the fact that they tend to have a symmetrical shape is just due to the physics of such a collapse.

Also, there is testimony from a Russian expert claiming that the 911 demolitions were effected by underground nuclear explosions underneath the buildings that collapsed. There are only a few buildings in the world that are rigged to be demolished by this method. It is a very plausible theory.

The physics of a sinkhole is similar. Something causes a cavity to form underground, then what is above it suddenly falls into it. The result is a hole filled to a greater or lesser degree with the rubble from the collapse.

You obviously haven't seen many of the recent perfectly round sinkholes that have appeared around the world over the last couple of years.