Two unlikely friends explore the abandoned metal mines hidden in the hills of mid Wales. Wales is known for its industrial past, with coal from the south and north east fuelling the world and slate from the north west roofing the world, but less is known about the metal mining industry that predates both industries by thousands of years. Steam train worker, author and historian Ioan Lord, and dairy farmer Al Tansey discovered one another through their shared passion for the past. Together, they are determined to ensure that this lesser-known part of Wales's heritage is not forgotten. After work at their day jobs, they head out in search of lost and abandoned metal mines in the area they call home, the hills of rural Ceredigion. With Ioan's obsession for history and his meticulous research, combined with Al's friendships with local landowners, they have unrivalled access to and knowledge of these underground mazes and the treasures they contain. This has led to a successful YouTube channel with nearly 40,000 followers and a substantial fan base in the USA. They have searched hundreds of mines, but none has rivalled the one they have nicknamed the 'Museum Mine', so-called because of the Victorian artefacts they believe may lie hidden beneath the surface. After clearing a new route into the mine, they're heading into it one last time to explore a section that has until now been inaccessible. For Ioan, this a deeply personal journey, as he believes one of his ancestors once toiled underground in this very mine. This is a risky business as they abseil, traverse and navigate the underground tunnels in one of their biggest adventures yet.