Scientists have discovered a crater on the moon that's bigger than the Roman Colosseum, the result of a powerful impact two years ago. Researchers reported Wednesday that it's the solar system's biggest known impact crater carved out in recent times. They say such a large-scale event is estimated to occur only once every 132 years, making this a treasure trove of information as NASA prepares to build a base for astronauts on the moon. NASA's Lunar Reconnaissance Orbiter spotted the crater after it was formed by a comet or asteroid fragment.