Discover the history of slate through the essential Slate Tour, descending into the Deep Mine. The tour includes walking and descending 61 well-lit, safe steps and takes you through the history of mining, starting 500 million years ago and culminating at the Industrial Revolution. By the 19th century, the slate industry in Wales employed an army of 17,000 men to extract half a million tonnes of slate per year and export it all around the world.