The brightest comet in the last quarter century -- since Comet Hale-Bopp in 1997 -- is visible from Northern hemisphere this summer. The Guardian offers stunning photos of sightings around the globe. Scientists say it's 3 miles (5 km) wide, and its nucleus contains sooty, dark particles from the birth of our solar system, 4.6 billion years ago. Comets are half water and half dust, and this one contains 13 million Olympic swimming pools of water. Best viewed an hour after sunset, just below the Big Dipper, here's where to look for it. After mid-August, it won't be visible for 6,800 years.