From the longer Wikipedia page 
In the first 50 years of the 19th century the population of London more than doubled from 1 million to 2.3 million. At this time all London's dead were buried in small parish churchyards, which quickly became dangerously overcrowded, leading to decaying matter getting into the water supply and causing epidemics. There were stories of graves being dug that already contained bodies, and bodies being flushed directly into the newly-built sewer system.
In 1832 Parliament passed a bill encouraging the establishment of private cemeteries outside London, and later passed a bill to close all inner London churchyards to new deposits.Over the next decade seven cemeteries were established:
- Kensal Green Cemetery, 1832
- West Norwood Cemetery, 1837
- Highgate Cemetery, 1839
- Abney Park Cemetery, 1840
- Nunhead Cemetery, 1840
- Brompton Cemetery, 1840
- Tower Hamlets Cemetery, 1841
In 1981 the architectural historian Hugh Meller dubbed the group of cemeteries "The Magnificent Seven" after the western film of the same name.