What is channeling in espresso and how do I prevent it?

Channeling occurs when water finds a weak path through the coffee puck and flows through it unevenly, extracting some parts heavily and leaving others under-extracted. Causes: uneven distribution before tamping, tamping at an angle, or cracks in the puck. Prevention: use a distribution tool or WDT tool to level grounds before tamping, tamp level with even pressure, ensure your basket is clean, and don't tap the portafilter after tamping. A bottomless portafilter makes channeling visible.