Will Millard returns with more of Wales's hidden wonders as he explores the forgotten, secret and almost inaccessible locations that reveal the country as you've never seen it before.

Will Millard returns with more of Wales's hidden wonders as he explores the forgotten, secret and almost inaccessible locations that reveal the country as you've never seen it before.

Ch1. Caves
Will explores some Wales's most intriguing and hard-to-access caves. From a smugglers' den to an ancient bone cave and from a revolutionary 19th century hide-out to one of the finest decorated caves in Europe, Will reveals the subterranean wonders of Wales you never knew existed.

Ch2. Copper
Will discovers the forgotten story of how Wales once led the world in copper production by visiting some extraordinary and abandoned locations. He begins his journey in Anglesey on Parys Mountain, once the world's biggest copper mine. Will goes beneath its Mars-like landscape into the miles of tunnels that were first constructed in the 18th century. Then in south Wales, Will unearths the remains of Swansea's once huge copper industry, tracking down the final relics of the lost industrial world of Copperopolis. The episode concludes with Will cave-diving through a 19th-century flooded mine to discover its important role in Wales's industrial revolution.

Ch3. Coal
Will Millard goes in search of the lost and hidden locations from Wales's coal history. Wales once led the world in the production of coal, exporting the black gold to the four corners of the Earth. Since that industry collapsed, much of the infrastructure has been destroyed, forgotten or hidden away. Will goes in search of those final pieces of coal history. He visits one of the country's last coal mines, where they have dug up centuries-old workings, explores the abandoned buildings of some of South Wales's forgotten collieries and lands on an island that was responsible for wrecking hundreds of coal ships.

Ch4. Slate
Will travels to North Wales to discover the forgotten history of the slate industry. Welsh slate once roofed the buildings of the British Empire and the industrial revolution. By the late 1800s, a third of all roofing slates on the planet came from North Wales. The industry produced communities, culture and an identity that are still alive today - but much of the history has been forgotten or vanished. Will tracks down some of the remains and relics from this vibrant period of industry by exploring deep inside the mountains of Snowdonia in some of biggest slate mines in the world.

Ch5. World War II
Will explores the hidden pieces of history from World War Two. Wales made a huge contribution to the war effort and the remote countryside was witness to many military activities. Remnants from that time can still be found but they lie hidden. Will seeks some of them out by visiting plane crash sites on the Brecon Beacons, uncovering an old training academy on the Great Orme, gaining exclusive access to an MoD training base that played a vital role in the war and tracks down the secret bunkers of a home-based army of saboteurs.

Ch6. Engineering
In this final episode Will gains exclusive access to explore the usually inaccessible locations that are home to some of the biggest engineering projects in Wales, both past and present. He kayaks over the highest navigable aqueduct in the world, visits the vital repair work to an underground Victorian engineering marvel, witnesses the defueling of Wales's last nuclear reactor and discovers how one of the biggest freight planes the world has ever seen is constructed.