Insane, intensely private, planet-engulfing nanite swarm.
The entire planet is covered in a beige haze of Nanites. It is unknown whether they were dropped as a weapon, were an out of control experiment, or are a means of extracting resources, but they have been thorough. There's nothing left but the planet's radioactive core. These Nanites are very efficient assemblers, and are eager to do so if provided raw materials. However the Nanites, are extremely protective of the secret of their design - they self destruct if any are captured or removed from the planet.