Inorganic crystalline life forms living beneath the sand of Velara III, which fought back when a terraforming project threatened their home.
The microbrains are inorganic, crystalline life forms found on Velara III. They live in a layer of saline water beneath the planet's surface and are linked together into a collective intelligence.
In Home Soil, a Federation terraforming team threatened their habitat without realizing they were alive. The microbrains struck back, killing a team member and seizing control of systems on the Enterprise, where they famously called humans ugly bags of mostly water. Picard negotiated a withdrawal, and Velara III was placed off limits for three centuries.