Engineer Tim Joyce explores a world-class engineering project, a power station hidden deep inside a granite mountain in County Wicklow. On top of the mountain, a man-made reservoir unleashes the power of this innovative pumped hydro-electric scheme built by Irish engineers in the late 1960s. Tim and the team of experts investigate how the reservoir feeds an underground hydro-electric power station – and vice versa. Turlough Hill Power Station was created in an area of environment and touristic sensitivity, in the days before renewable energy systems even entered the vocabulary.