Tunnelling is one of the most common complaints we hear about from UK candle owners, and it is almost always down to one thing: the candle was not given a long enough first burn. On the first light, a candle needs to burn until the melted wax pool reaches all the way to the edge of the container, which on a standard 8cm jar usually takes two to three hours. If you blow it out before that happens, the candle "remembers" that smaller diameter and will keep tunnelling down the middle on every burn after. The fix for a candle that has already started tunnelling is to even it out: wrap the outside of the jar in foil to trap heat and force the wax pool wider, or carefully trim away the raised wax walls with a knife once the candle has cooled. Going forward, always allow the full first burn, and keep the wick trimmed to about 6mm before every relight.