In this three-part BBC Gaeilge series, Cormac Ó hÁdhmaill takes us on an adventure underground as he explores the unique spaces hidden beneath our feet, from souterrains to nuclear bunkers and abandoned railway tunnels. In this episode, he explores the sea caves of the Antrim Coast and Rathlin Island, and travels to the mystical Pollnagollum Cave in Fermanagh, an early Victorian precursor to the Marble Arch showcaves.