With more than six million kilometers of highways and 240,000 kilometers of railways snaking across the United States, life above ground has become increasingly congested. Tunnels provide some of the last available space for cars and trains, water and sewage, even power and communication lines. Today, it's safe to bore through mountains and burrow beneath oceans -- but it was not always this way. In fact, it took engineers thousands of years to perfect the art of digging tunnels

Before cars and trains, tunnels carried only water                            

   Image of a Roman Aqueduct                         

Roman engineers created the most extensive network of tunnels in the ancient world. They built sloping structures, called aqueducts, to carry water from mountain springs to cities and villages. They carved underground chambers and built elegant arch structures not only to carry fresh water into the city, but to carry wastewater out

Worsely underground canal tunnel                                    Image of Worsley Underground Canal                     

By the 17th century, tunnels were being constructed for canals
Without roads or railways to transport raw materials from the country to the city, watery   highways became the best way to haul freight over great distances 

 

Inauguration of Holland Tunne l
New York, New York
Image of Holland Tunnel
With trains and cars came a tremendous expansion in tunnel construction
During the 19th and 20th centuries, the development of railroad and motor vehicle transportation led to bigger, better, and longer tunnels

 

Tunnel boring machine
Used to carve New York Third Water Tunnel
Image of Tunnel Boring Machine 

Today, not even mountains and oceans stand in the way.With the latest tunnel construction technology, engineers can bore through mountains, under rivers, and beneath bustling cities. Before carving a tunnel, engineers investigate ground conditions by analyzing soil and rock samples and drilling test holes

TO BE CONTINUED