Potatoes

It's either slugs or more likely wireworm. If the haulms have yellow leaves at the bottom and the roots have little cysts on them, it could be potato eelworm but I hope not.

It won't be potato eelworm, which doesn't burrow into the tubers, although it severely weakens or even kills the plant and reduces yields, although what you do manage to harvest is still edible.

As GG says, most likely slugs or wireworms - you may find a tiny black slug inside one of the tunnels, although yours look as if they have tried to 'heal' so the damage may have been done some time ago. Using slug bait during planting and earthing up may help, although not everyone would be happy with doing that.

Wireworms are more common when spuds are grown in soil which was recently used for growing grass, and the problem should reduce in future years.