Candle tunnelling is the phenomenon of candles burning down in a tunnel around the wick, rather than burning down evenly across the entire top surface.
Tunnelling occurs if, the very first time the candle was burned, it wasn’t allowed to melt all the wax on top. This creates a “memory ring” in the wax. The candle will subsequently tunnel down this ring.
The solution, therefore, is to make sure the ‘first burn’ is done correctly. According to osmology.co:
How long this takes depends on the diameter of the candle.
I realize now that I’ve been burning candles wrong my entire life.