Mineshaft

Abandoned Mine Shafts are a new type of structure that was added in Beta 1.8, along with strongholds.

Structure
Abandoned mine shafts are a series of straight intersecting tunnels 3 blocks wide. They have uneven ceilings with a height of 2-3 blocks. Wooden planks and fences make up the supports of these tunnels. Incomplete sections of minecart tracks can be found at the floor of these shafts and torches are occasionally placed on walls. Cobwebs can be found at random. Cave Spider spawners surrounded with cobweb can be found. Occasionally, stair-like tunnels can be found.

Cave Spiders are exclusive to these areas. There are chests spread throughout the mine that contain things such as Gold and Iron ingots, ores, food or seeds. They can even contain Diamonds or pickaxes and may sometimes spawn in air as a world generation glitch. It is possible but rare to have it spawn in an ocean so the shaft is flooded. Abandoned Mine Shafts are also rich with ores such as Iron, Gold, and rarely Lapis Lazuli and/or Diamonds. As the 'shafts' are made of Wooden Planks, Abandoned Mine Shafts actually in turn provide very good source of wood while prospecting underground.

Obtainable items:
 * Diamond
 * Coal
 * Lapis Lazuli
 * Redstone Dust
 * Iron Ingot
 * Gold Ingot
 * Melon seeds
 * Pumpkin Seeds
 * Bread
 * Rails
 * Iron Pickaxe

Exploration
Abandoned Mineshafts may be large, and may be akin to labyrinths during exploration, making it difficult to keep bearings. The venomous Cave Spiders are very common and make exploration dangerous unless prepared. However, due to placed torches, the presence of other mobs is much lower so areas not near spider spawners are much safer than caves. When gravel is encountered at the end of a shaft, sometimes there is more behind the gravel. If you find such a wall, dig a few blocks past and check for the wooden frames.

Minecart tracks may be generated floating in the air from a world generation glitch. These tracks behave similarly to floating snow in that all adjacent glitched blocks will pop out when one is mined.

Due to the large amount of resources in Abandoned Mine shafts, a player could simply cut down cobwebs with a sword, knock out sections of wood, and create a bed,(assuming it is nighttime and the player has made a safe room for a bed) making navigation safer because of the very labyrinth-like structures and dangerous spider spawners. However, if the player was lost, they might lose any items that they wanted to store in a safe room. Use extreme caution when exploring these areas, for the many riches found in these areas often distracts players from the danger that lurks around them...

Trivia

 * In the 1.8 pre-release 1 and 2, support beams in mines were only made of planks. Fences were added later in the 1.8 version.
 * Some spots in/around the mine shafts have been known to have a strange terrain generation that causes a random 'room' in the mine with a dirt flooring, usually fairly large, with no observable purpose. (Much like circular chambers in cave systems from the generation code.) Whether this is a bug or intended is unknown.