Bored through mountains or burrowed beneath oceans, tunnels provide spaces for cars and trains, water and sewage, and power and communication lines. Although tunnels have existed for thousands of years Roman engineers created the most extensive network of tunnels in the ancient world they have been perfected by today’s technology. To build a stable tunnel, engineers dig through the earth, or excavate, using special tools and equipment. If the ground is unstable, engineers must support the ground around them while they dig. For the support work, they often use a tunnel shield, a cylinder pushed ahead of tunneling equipment to provide advance support for the tunnel roof. For harder, mountainous rock, engineers use humungous rock-breaking devises called tunnel-boring machines. Once the tunnel is structurally sound, engineers line tunnels with final touches, like the roadway and lights. If builders are working underwater, pre-made tunnel segments are often floated into position, sunk, and then attached to other sections.