Water seeping through soil into bedrock dissolves the rock along cracks and between layers (called bedding planes). As the cracks get bigger, soil falls into them. Water flowing underground may also dissolve and remove earth materials from below, so the land surface forms a bridge or arch of soil over a hidden hold or cave.
The soil arch gets thinner and weaker as more earth materials collapse into the buried hole or cave. The collapsing material may fill up the hole and stop the erosion. But it it's removed from below by flowing water the collapsing continues right up to land surface.
Circular cracks appear in the soil at the surface just before a catastrophic collapse of the surface.
Water washing down the sides of the sinkhole soon erodes the sharp edges into the smooth contours of the classic bowl-shaped sinkhole.