The Shellworlds were mostly hollow. Each had a solid metallic core fourteen hundred kilometres in diameter. Beyond that, a concentric succession of spherical shells, supported by over a million massive, gently tapering towers never less than fourteen hundred metres in diameter, layered out to the final Surface. Even the material they were made from had remained an enigma – to many of the galaxy’s Involved civilisations at least – for over half a billion years, before its properties were fully worked out. From the start, though, it had been obvious that it was immensely strong and completely
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