People have always been dreaming of going to the moon. As long as the year 1901, H. G. Wells, an English writer, wrote a book vividly describing a trip to the moon. On landing there, the explorers discovered that the moon was full of underground cities. Words failed to express their surprise when they spotted so many "moon people". The "moon people" felt even more surprised. "Why," they asked, "are you traveling to outer space when you don't even use your inner space?" H. G. Wells could only imagine a travel to the moon. In 1969, human beings really left their footsteps on the moon. People today know that there are no underground cities on the moon. However, the question that the "moon people" asked is still an interesting one. A growing number of scientists are seriously thinking about _ . Underground systems are already in place. Many cities have underground car parks. In some cities, such as Tokyo, Seoul and Montreal, there are large underground shopping areas. The "Channel", a tunnel connecting England and France, is now complete. But what about underground cities? Japan's Taisei Corporation is designing a network of underground systems, called "Alice Cities". The designers imagine using surface space for public parks and using underground space for flats, offices, shopping, and so on. A solar dome   would cover the whole city. Supporters of underground development argue that building down rather than building up is a good way to use the earth's space. The space, they say, can be used for farms, parks, gardens, and wilderness. H. G. Wells' "moon people" would second it. Would you? The explorers in H. G. Well's story were surprised to find that the "moon people"  _  .
knew so much about the earth understood their language lived in underground cities were advanced in technology