Imagine visiting a far distant galaxy and addressing a postcard to your loved ones back home. You might begin with your house on your street in your hometown, somewhere on Earth, the third planet from our sun. From there the address could list the sun's location in the Orion Spur, a segment of a spiral arm in the Milky Way's suburbs, followed by the Milky Way's residence in the Local Group, a gathering of more than 50 nearby galaxies spanning some seven million light-years of space. The Local Group in turn exists at the outskirts of the Virgo Cluster, a 50-million-light-year-distant cluster of more than 1,000 galaxies that is itself a small part of the Local Supercluster, a collection of hundreds of galaxy groups sprawled across more than 100 million light-years. Such superclusters are believed to be the biggest components of the universe's largest-scale structures, forming great filaments and sheets of galaxies surrounding voids where scarcely any galaxies exist at all.