Old-time pit ponies line up to be harnessed before being taken down an underground coal mine. While their holders are chatting, a bit of mischief is about to occur. The ground below them is depicted in cross-section: a large coal seam is visible. The crossection is depicted in acrylic, which allows for quite a bit of texture including use of bubble-wrap. Upper part is done in watercolour for a finer effect. The figures were drawn in Indian ink with a post office pen before being painted. I live in a coal-mining district in Australia and worked as a fossil fuels geologist.