I like to distinguish between bubble pops and bubble pop craters. They both start with bubbles, but bubble pops happen when the glass is still hot and a subsurface bubble burst - they have smooth, rounded edges. Bubble craters happen when the bubble is subsurface and is initially covered by a thin pane of glass after it cools. At some point, the pane shatters and you're left with a crater that has rough edges and a smooth, rounded bottom.