This passage is mainly about______.

Over 2,000 years ago, Rome was the center of a large empire. The Romans needed a way to move their large armies quickly so that they could protect their huge country. They needed land trade routes. So they joined all the parts of their empire by a network of roads.
    Beginning in 300 B.C., the Romans built roads in Europe, Asia, and North Africa. By A.D. 200, they had built 50,000 miles of almost straight highways.
    To build their fine highways, the Romans began by removing all soft soil. They dug until they reached hard ground. Then they added layers of stone and cement. The most important roads were paved with large, fiat stones. Main Roman roads were sometimes as wide as our three-lane highways.
    To build their roads, the Romans sometimes had to dig tunnels through mountains. They did not have explosives to cut through rocks. They heated the rock with fire and then threw cold water over it. When the rock cracked, they dug it out with hand tools.
    Roman soldiers and slavers built the roads without powerful machines. They had only simple tools, such as shovels and picks. Yet the roads were so well built that they were used for hundreds of years.