Stars are organised into galaxies, which in turn form clusters and superclusters that are separated by immense voids. Prior to 1989, it was commonly assumed that virialized galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction. However, based on redshift survey data, in 1989 Margaret Geller and John Huchra discovered the "Great Wall," a sheet of galaxies more than 500 million light-years long and 200 million wide, but only 15 million light-years thick. The existence of this structure escaped notice for so long because it requires locating the position of galaxies in three dimensions, which involves combining location information about the galaxies with distance information from redshifts. In April 2003, another large-scale structure was discovered, the Sloan Great Wall. However, technically it is not a 'structure', since the objects in it are not gravitationally related with each other but only appear this way, caused by the distance measurement that was used. One of the biggest voids in space is the Capricornus void, with an est. diameter of 230 million light years[2]. However in August 2007 a new supervoid was confirmed in the constellation Eridanus, which is nearly a billion light years across.[3] Originally, it had been discovered in 2004, and was known as the 'WMAP Cold Spot'.