While not all manmade tunnels are tiled, this type of surface is used for underground roadways and railways all around the world. As it turns out, there are some very good reasons for this.
The main reason why tunnels are tiled is the same reason why most showers are tiled. Smooth ceramic tile is fairly easy to clean because particles of dirt and grime generally don't get imbedded in the material. Instead, most particles collect on top of the tile, and will wash away with detergent and water. This resistance to grime (as well as to moisture) makes tile an extremely durable and easily cleaned material. Even under harsh conditions, it takes a long time for tile to deteriorate, and it's pretty easy to get all of the grime off.
This is especially important in roadway tunnels because of all the exhaust emitted by the cars passing through. These fumes, along with any dirt or roadway salt kicked up by the cars' tires, gets trapped inside the tunnel, leading to a constant accumulation of gunk on the walls. When things get too dirty, city workers can use high-pressure water hoses or extendible brushes to clean off the tile.
Tiles also work well in tunnels because you can easily arrange them to cover pretty much any shape. If you had a cylindrical tunnel, for example, you could attach individual tiles all along the wall and ceiling as if the surface were straight. All of the individual tiles would be flat, but each one would sit at a slightly different angle than the tiles below it and above it. Overall, then, the flat tiles would form a curved surface. To cover the same space with a metal, you would need to prefabricate a metal sheet so that it curved in exactly the same way as the walls and ceiling. You would also need to design metal sheets to fit around any unusual ledges or corners, while tile would adapt easily to these surface areas.
Tiles are also easy and inexpensive to replace should they become worn or damaged. Instead of re-installing an entire sheet of metal, you can simply pop out any busted tiles and put in new ones. It's also simple to see when a tile needs to be replaced, even from a good distance; the tile will have obvious cracks in it. This makes it much easier to fix any damage to the tunnel surface before it becomes a problem.
Tile is also a good reflective surface. In roadway tunnels, this makes for safer driving conditions using less artificial lighting. Scattered fluorescent lights, as well as the cars' headlights, reflect light off the walls, illuminating all areas of the tunnel. This is just an added bonus, of course: There are many other materials that reflect light more effectively, but they are not nearly as durable.
Here are some interesting links:
- Tunnel Design and Construction
- New York Underground
- The Big Dig Project
- Why do they use salt to melt ice on the road in the winter?