Near the Pleiades Cluster
In the mid-24th century, after assessing the planet was lifeless, the Federation's Terraform Command established the Velara Base on Velara III. This base was responsible for a multidecade terraforming project with the aim of establishing a M class environment on this planet.
The terraforming was, however, canceled in 2364, when it was discovered that the process was killing silicon-based lifeforms, dubbed as "Microbrains", in a thin layer of water between two planetary layers.
Following the discovery of the "Microbrains", and the retaliatory death of Hydraulics Specialist Arthur Malencon by these lifeforms, an agreement between the little crystals and Captain Picard of the USS Enterprise-D placed Velara III under a 300-year quarantine. (TNG: "Home Soil")