In Japan's 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 go from one place to another. The price of the land, too, has skyrocketed. All these 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 shopping centres. What are some of the advantages of shopping and eating underground? Clean air is one of them. The city of Tokyo has one of the most serious smog problems in the world. Another advantage is that you escape the ever-present threat of traffic accidents on the city's busy streets. Still another is the convenience of getting around: you are usually right next to, or even in, a subway station. And you can even spend the night underground if you like. The Kibosh station, for example, in downtown Tokyo, has a hotel with a bar, restaurant, and barbershop. What's the meaning of "skyrocket"? It means "  _  ".
go up sharply be as high as that of a rocket fall rapidly be as low as that of a rocket