Ulfstead Mine

Ulfstead Mine, also referred to as Stephen's Mine, was introduced in King of the Railway. It is located beneath Ulfstead Castle and Stephen became trapped in it after he had to bash through the blocked entrance to avoid a collision with some trucks. King Godred's long-lost crown, which was stolen a long time ago, was discovered here by Stephen.

In the eighteenth season episode, Missing Gator, the entrance to the mine was reboarded up until some runaway trucks bashed through the blocked entrance and into the mine where Percy came to rescue them.