Look at the picture representing three way tunnels, and tell me which one is dug horizontally?
Three ways to make tunnels through the mountains.
No top and no bottom, and middle, running in an arc, which in all points forms right angles with the direction of vertical lines (or earth radii). This is a horizontal tunnel, his curve corresponds with the curvature of the earth.
Large tunnels are breaking commonly as shown above: in a straight line, tangent to the earth's surface at the endpoints of the tunnel. This first tunnel goes a bit up and then down. He is the convenience that water does not stagnate in it, and she runs down to the edges.
If the tunnel was dug horizontally, then a long tunnel would have an arched shape. The water would not have aspirations to flow from him, as each point would be in equilibrium. When such a tunnel longer than 15 km (Simansky, for example, has a length of 20 km), standing at one output, you cannot see the other, the line of sight rests on the ceiling, as the midpoint of such a tunnel more than 4 m rises above its endpoints.
Finally, if you dig a tunnel in a straight line, connecting the extreme points, he will be at both ends to have a slight tilt down the middle. Not only does water will not drain from him, but instead accumulate in the medium, the lowest part of it. But, standing at one end of this tunnel, you will see another. The accompanying drawings illustrate this *
* From the above it follows, among other things, that all horizontal lines - curves; straight horizontal lines can not be. Vertical, in contrast, can only be straight.