In Japans capital city of Tokyo, earthquake danger limits the height of buildings. The city has spread out so far and the traffic has become so heavy that it is very difficult to get from one place to another. The price of land, too, has become very high. All this explains why a group of Japanese land developers came to the conclusion that there was nowhere to go but down. So far they have dug out space underground for fifteen major shopping centers, and the underground construction has only begun. What are some of the advantages of shopping and eating underground? Clean, filtered(过滤的)air is one of them. The city of Tokyo has one of the most serious smog problems in the world. Another advantage is that there wont be traffic accidents as that on the citys busy streets. Still another is the convenience of moving from place to place. Youre usually right next to, or even in, a subway station. And you can even spend the night underground if you like. The Kyobashi Station, for example, in down town Tokyo, has a hotel with a bar, restaurant and barbershop.

It is very difficult to get from one place to another in Tokyo because______.