The recent sink hole isn't actually a sink hole.
The top geologists were saying it is a volcanic piping feature. The area the city is built on is a huge thick layer of volcanic ash or pumice.
Essentially 35-45% of the city is on it, and will have to be relocated eventually.
They claimed that due to this, there will certainly be many more cave ins randomly in the future. The water run off from sewage, rain, etc, was
causing the ash to collapse through or something.
But in this particular case, the reason it was circular is because it was a volcanic piping feature.
This is why there were very straight clean walls inside the feature. Because it is a ancient volcanic pipe. (Or something?)
This is not the original link I learned this from, but this is a link discussing it so it's worth a read.