In this novel, Isaac Asimov first introduced Elijah Baley and R. Daneel Olivaw, who would later become his, and more so his readers', favorite protagonists. They live roughly three millennia in Earth's future, a time when hyperspace travel has been discovered, and a few worlds relatively close to Earth have been colonised ? fifty planets known as the "Spacer worlds." The Spacer worlds are rich, have low population density (average population of one hundred million each), and use robot labour very heavily. Meanwhile, Earth is overpopulated (with a total population of 8 billion), and strict rules against robots have been passed. The eponymous "caves of steel" are vast city complexes covered by huge metal domes, capable of supporting tens of millions each. The New York City of that era, for example, encompasses present-day New York State, as well as large tracts of New Jersey. Asimov imagines the present day's underground transit connected to malls and apartment blocks, extended to a point where no one ever exits to the outside world. Indeed, most of the population cannot leave, as they suffer from extreme agoraphobia. Even though the Robots and Foundation series were not supposed to play in the same universe until much later, those 'caves of steel' could easily be construed as a foreshadowing of the planet Trantor. In The Caves of Steel and its sequels, Asimov paints a grim situation of an Earth which has become pseudo-socialist to deal with an extremely large population, and of luxury-seeking Spacers who limit birth so that each may have great wealth and privacy. However, Asimov (who was agoraphobic) did not find the lack of daylight grim: one of his anecdotes tells how a reader asked him how he could have imagined such an existence with no sunlight. He relates that it had not struck him till then that living perpetually indoors might be construed as unpleasant.