What is tunnelling in a candle and how to avoid it?

Tunnelling is when a candle doesn't burn evenly and so it creates a hole or 'tunnel' in the candle. Tunneling will create a hole and each time you light the candle the hole will become deeper. This will reduce the lifespan of the candle and the visual appearance.  

To avoid tunneling the best tip is to make sure you burn the candle for at least 2 hours or until the wax is melted all the way to the edge.

If the wax is still not melted to the edge after about 2-3 hours it could be that the diameter of the wick is too small. In this case you will need to use a bigger sized wick or several wicks to avoid tunneling.