On March 25, 1655, a Dutch scientist named Christiaan Huygens pointed his homemade telescope at Saturn and spotted something new. He had discovered Titan, the largest moon of Saturn. Huygens built his own telescopes because the ones available were not powerful enough. His lenses were some of the best in the world at the time. Titan turned out to be an extraordinary world. It is the second largest moon in the entire solar system, even bigger than the planet Mercury. What makes Titan truly unique is its atmosphere. Titan is the only moon known to have a thick atmosphere made mostly of nitrogen, the same gas that makes up most of Earth's air. The atmosphere is so dense that you cannot see Titan's surface from space. In 2005, a spacecraft called Huygens, named after the discoverer, landed on Titan. It sent back pictures of a surface covered in orange sand dunes and lakes filled not with water but with liquid methane. Scientists believe Titan may hold clues about what early Earth looked like billions of years ago, making it one of the most fascinating places in the solar system.