The commentary doubles as a synopsis, and is reproduced below:

In South Wales, in seams over two and a half feet thick, there is still at least ten thousand million tons of the coal that made her famous - enough for another 400 years.

The map will show where the best workable reserves lie. The dotted line is the boundary of the coalfield. Of anthracite there is a wide unworked belt stretching from above Llanelli to the Vale of Neath. There are large reserves of steam coal in the Rhondda and around Treharris. The Rhondda coal lies deep under the mountains above Maerdy. Along the southern boundary of the coalfield are belts of good coking coal mixed with less valuable coal. These belts run from Pontypool to Port Talbot.

Let's look underground here. We'll draw a line through Nantgarw and look at a cross-section of the ground beneath it. These seams are good coking coal. Nantgarw is an old pit reborn. During the last year or two new coke ovens, washeries and power house have been growing up. Underground, the seams tilt steeply towards the surface. That's why there's so much left: it's hard to work.

Horizon mining is the answer. First, shafts are sunk. We're only showing one to make things simpler. At 280 yards down, a level road is driven across the seams of coal. At 380 yards, another. Men and materials go in by along the top road, coal comes out by along the bottom road, hauled by diesel locomotives. Between the roads, the coal can be worked as a longwall face.

Of course, the model has oversimplified the story. But Nantgarw could not produce this good coking coal economically without horizon mining. The South Wales coalfield will be turning out best Welsh for a good few hundred years yet.