In a special programme showing on St. David's Day, we celebrate the awesome beauty of some of Wales' iconic engineering achievements, structures which help define the landscape of Wales. The Pontcysyllte Aqueduct in the Dee Valley is a masterpiece of engineering. Completed in 1805, Thomas Telford's design was extraordinarily ambitious and marked the beginning of a new confidence in 19th century engineering. Eddie Butler tells the story of the acqueduct, along with three other great feats of engineering in Wales, namely the Severn Tunnel, the Menai Suspension Bridge and Snowdon Mountain Railway. Featuring experts, the programme looks at the brilliance of these structures.