At one time, the world's ships and trains created a huge demand for steam coal – and the best coal in the world was in Cwm Rhondda – one of the South Wales Valleys. Dozens of pits opened, and thousands of workers flowed into the area. They created massive wealth . . . but not for everyone.

Ar un adeg, roedd llongau a threnau'r byd yn creu galw mawr am lo ager - ac roedd glo gorau'r byd yng Nghwm Rhondda - un o gymoedd De Cymru. Agorodd dwsinau o byllau glo a heidiodd miloedd o weithwyr i'r ardal. Fe wnaeth y pyllau glo greu cyfoeth enfawr ... ond ddim i bawb.