A young woman drowned and a man is missing after they were caught inside Prague's drain system by torrential rains while participating in a global GPS-based treasure hunt, police said Sunday. They were among a group of four people "geocaching" - using their smartphone's GPS to search for little treasures hidden all over the world - when the rapidly rising water from the storm trapped them in the Czech capital's underground drainage system on Saturday night. "A woman was found dead in the Vltava (river) and a man is still missing," police spokesman Jan Rybansky told AFP.