A "tunnel" forms when the middle part of a candle melts and the edge does not. The candle therefore only burns in the middle and leaves the edge standing. This can result in the wick soon disappearing into the wax as the candle burns futher into the centre. Your candle burns less than intended.

So, what can you do to avoid candle "tunneling":

1. If the "tunnel" is not too deep, let your candle burn constantly 3-4 hours to generate heat what will melt the edge. 

2. If you have very light "tunneling", you can use a hairdryer to melt the wax on the outside an thus to avoid "tunneling". 

3. If the "tunneling" is very severe and your wick has almost disappeared, you can wrap alumium foil around your candle, leaving just a little space at the top for the candle to light and air. The heat cannot escape and causes the wax to melt at the edge. 

Try out the techniques and see what it does to your candle!