There are three different kinds of tunnels. Mine tunnels go deep down into the earth to enable miners to get to mineral or metal deposits (and back to the surface). These tunnels are cheaper to build than tunnels for transportation. They are also not as safe. This is because from the start they are not designed as permanent tunnels. Other tunnels are built specifically for water, sewage or gas. These are called tunnels for public works. They are built to transport these materials over long distances.
The third kind of tunnel is for transportation, like for trains and cars. Transportation tunnels have to go through mountains, underground or under bodies of water. Planning a tunnel is complicated, and it uses science and mathematics. The way a tunnel is built depends on what material the tunnel has to go through. Building a tunnel through anything is a slow process that involves excavation by digging, blasting or boring into rock.