Vouliagmeni Lake

This guide to Vouliagmeni Lake near Athens starts with the warm, fish-nibbled water swimmers wade into, then traces how the lake formed when an underground hot-spring cavern collapsed. Past the sunlit shallows, its floor opens into a flooded tunnel system cave divers have followed for kilometers into the mountain without finding an end, including amateur divers who went looking in the 1980s and 90s and never returned. An international team of Swiss, Italian, French and Greek cavers spent years mapping the maze before anyone could chart it safely.