Sinkhole

A sinkhole is a dip, crater, or hidden air pocket that forms in the wax as a candle cools, most often in the center near the wick or just beneath the surface. It happens because wax contracts as it cools, and in a thick or fast-cooling pour the outer layer can set before the center has finished shrinking, leaving a gap.

Sinkholes are mainly a cosmetic problem, since a small pocket beneath the surface does not usually stop a candle from burning normally, but a visible dip on top looks unfinished on a candle meant for sale or gifting. Larger or deeper hidden pockets can occasionally affect how evenly the wax melts around the wick as the flame reaches that area.

A common fix is a second, smaller top-up pour once the first pour has mostly set but is still slightly warm, filling in any dip before the candle fully hardens. A common mistake is pouring a large candle in one thick layer and assuming it will cool evenly, which is when sinkholes are most likely to appear.