Mark O’Neill is an English photographer who specialises in light painting in abandoned places. Having spent his career travelling, Mark has explored unique, unseen spaces in different countries around the world, capturing everything from Victorian tunnels in Sheffield to wartime bunkers in France and Germany.
Mark's project Underground Spaces features eye-catching images that took him to unforgettable places such as reservoirs and sewers beneath the city of London and bunkers that still remain underneath the Maginot Line in France. 
In terms of exploration, Mark said that one of the most unforgettable journeys was to an airbase in Željava, Croatia, where he spent three days underground at the $6 billion facility bored into the base of Plješevica Mountain, straddling the border between Croatia and Bosnia and Herzegovina.