The One-Million-Liter Test Sphere — also known as the Test Sphere, the Horton Test Sphere, the Cloud Study Chamber, Building 527, and the “Eight Ball” (or “8-ball”) — is a decommissioned biological warfare (BW) chamber and testing facility located on Fort Detrick, Maryland, USA. It was constructed and utilized by the U.S. Army Biological Warfare Laboratories as part of its BW research program from 1951 to 1969. It is the largest aerobiology chamber ever constructed and was placed on the National Register of Historic Places in 1977.