Phage Rock is one of the Culture's oldest habitats. It had started out as a three-kilometre-long asteroid, orbiting in one of the solar systems that would later become founding members of the Culture. Altered through technology, it became a travelling habitat, fitted with warp and hyperspace engines, and armed with weapons. Sentient, it became one of the first space-based entities to join the Culture. Through the millennia, it has grown to just over seventy kilometres long and is home to one hundred and fifty million people.