A colossal abandoned shell built around a star, on whose surface the transport Jenolan crashed with Montgomery Scott aboard.
The Dyson sphere encountered by the Enterprise is an immense artificial structure enclosing a star, with an outer shell large enough to hold the surface area of millions of planets. Its builders appear to have abandoned it long ago, as the star within had grown unstable and dangerously radioactive.
In Relics, the Enterprise traces a distress call to the USS Jenolan, missing for 75 years and crashed on the sphere's exterior. Inside, Montgomery Scott has survived in a transporter buffer. When the Enterprise is pulled through a portal into the sphere, Scott and La Forge wedge the Jenolan in the closing hatch so the ship can escape.