Lakeman
This are many things in nature that can make a round hole:
. Water-ware where a small pebble is caught up in an indention and is swirled around and around eating out the softer material making a hole.
. A bubble that forms in mud, then the mud hardens around it and the mud then turns to stone.
. A root that has rotted away leaving a space and the soil around it turns to stone leaving a hole.
. Volcanic lava bubbles and holes are left from this action.
Jack