What is paramount for securing a yard against a Lakeland Terrier's instinct to dig?

Answer

A fence that extends below the ground line or has a secure apron

Because they possess a strong instinct to burrow after quarry, security requires a fence that is physically secure underground or has an apron to prevent them from digging underneath.

What is paramount for securing a yard against a Lakeland Terrier's instinct to dig?
behavioranimalDogbreedTerrier