Mineshaft

Abandoned mine shafts are naturally generated structures that generate in the Overworld. They were added in the adventure update, along with strongholds and NPC villages.

Structure
Abandoned mine shafts consist of:
 * A starting point - a large room with a flat dirt floor, an arched ceiling and up to four exits (one in each direction).
 * Corridors - 3×3 block corridors featuring supports made from fences and Wood Planks (sometimes complementary with torches), broken rails, minecarts with chests (containing resources) and cave spider spawners surrounded by cobwebs.
 * Crossings - intersections with wooden pillars, sometimes two floors tall.
 * Staircases - diagonal sections of tunnel.

The generator continues to add pieces onto an exit until it reaches a "depth" of 8 (eight pieces from an exit).

Abandoned mine shafts are generated before chunk features such as ores. As with other features, multiple abandoned mine shafts can generate near each other and intersect, producing larger complexes. They can also intersect with other cave systems, notably ravines. Corridors will generate a floor of wooden planks if they cross over empty space.

Obtainable items
Mining of ores spawned in the side of the shaft are obtained, as well as mob drops.

Chest contents
Each chest contains 3-6 stacks of items, chosen from the following list. Chance listed is per stack, not for the chest as a whole.

Exploration
Mine shafts can be easily found in ravines or large caves, (many times on some occasions) where it is likely that a mine shaft will intersect them. In ocean biomes it is possible to find the upper part of the shaft complex exposed to water at some of the sections.

Abandoned mine shafts may be large and be akin to mazes, making it difficult to keep bearings. The venomous Cave Spiders are common and make exploration dangerous unless prepared. However, due to naturally generated torches, the presence of other mobs is much lower, making areas aside from the Cave Spider spawners safer than caves.

Gravel sometimes collapses into shafts, making them appear to be dead ends. Usually the shaft continues 1-3 blocks past the gravel, so be sure to check if it really is a dead end or not.

Using the large amount of resources found in abandoned mine shafts, the player may cut down cobwebs to collect string (then craft that into wool, fishing rods, leads and bows), take out wood and fence posts from the supports, and create a safe room with a bed within the shafts. (This lets you respawn nearer your death, making it much more likely to recover your possessions.) It's also a popular choice to include a crafting table and a furnace in the safe room. This makes exploration easier, although becoming lost means losing resources within the room and possibly death from the Cave Spiders or other hazards within the elaborate structure. One should put caution before anything else when exploring these locations. If the need of food arises, melon seeds and wheat are found in loot chests and water can be found in caves around the mine. It is also likely to find bread in the chests as well. Also, if you don't feel like enduring the monster encounters or worrying about hunger, you can always set to peaceful mode.

The minecart tracks can be repaired, and you can collect them by hand, pickaxe, or pour a bucket of water on top of them. The mine shaft can be used for mining of your own, after it is cleaned up and rid of mob spawners.

How to explore
Abandoned mine shafts can be extremely difficult to explore, especially since some can be cut in half through ravines, though this is made easier by the bridges added in 1.2. If a ravine is present, adding bridges to cross it, or waterfalls to get up and down, can help deal with the obstacles. You will likely need to build bridges or staircases to access all the areas within an abandoned mine shaft—valuable blocks and chests can often be found in hard-to-reach areas.

As with most underground areas, your first priority should be to make sure the whole area is well lit—there will be torches already present, but these will be somewhat uneven, leaving many dark areas for you to light. While you're at it, note any minerals present like iron ore, or coal, which may be embedded in the wall. Having spare tools, weapons, and other equipment is also helpful. Shears are very useful for dealing with cobwebs without dulling your sword, and unlike most caves, you'll want axes for the fences and beams. A Flint and Steel can also be helpful in coping with cave-spider spawners. It is also common for mine shafts to intersect ravines, caves, and other underground structures—these can be blocked off for later, or explored along with the mineshaft. Both water and lava can be found in mineshafts and connected caverns.

Because of the labyrinth-like characteristics of abandoned mine shafts, it is important to leave pointers to where the exit routes are. The tactics in the Navigation tutorial are helpful as always—it's probably best to use something besides rails or torches as trail markers, as both are found naturally in the mineshafts. Wool can be made from cobweb string, and if Lapis Lazuli ore is present, you can make blue wool (and light blue as well, with bone meal from skeletons). All three wool colors make fairly distinctive markers. Blocks of dirt can also be used to mark off explored areas, or they can be blocked off entirely with fences harvested from the supports. A slower but productive tactic is to clear out the shafts entirely as you go, mining fences, beams, rails, cobwebs, and even "stray" stone blocks from the ceiling, to leave empty 3&times;3 corridors (with occasional chests full of mined stuff). This makes it dead obvious where you've been, while providing lots of resources and removing various obstacles.

Cave spider spawners
These are a special hazard of abandoned mineshafts. Unlike the spawners found in dungeons, they will not be neatly enclosed in cobblestone rooms, nor do they come with treasure chests. Instead, the "lair" for the spawner consists of a broad swath of cobwebs (usually filling a corridor), making it quite hard to reach and destroy the spawner. The cave spiders, of course, are unaffected by the cobwebs, and will happily crawl along walls, mine supports, and other fixtures to reach you. There are often two or more lairs near each other (sometimes there will be two or more side-by-side), so don't let down your guard just because you've destroyed one. They can also appear embedded in hard to reach places like ceilings, bridges, or high intersections, or behind small holes, from which they can drop a steady supply of spiders when you don't expect it.

To deal with these, you need tools to get through those webs. Aside from your usual weapons, shears will save wearing down your sword (that's a lot of webs), and flint and steel will set fires among the webs, burning some away, providing light, and scorching any spiders that do spawn. A water bucket may be able to clear out the bottom layer of webs (and fend off some spiders), but will also take out any floor torches. Once you've spotted a cave spider lair, place warning markers in case you need to beat a hasty retreat. If it's in a corridor, you may want to add fence gates on nearby mine supports, blocking off the corridors nearby so as to choke off the spiders' approach. (Remember that unlike regular spiders, cave spiders can fit through 1&times;1 holes.) Approach with caution, killing spiders with bow or sword as they emerge from the webs. If you get poisoned, retreat to recover and heal yourself (Golden Apples and milk buckets are helpful). When you reach the webs, alternately plant torches (to cut down on spawning), set fires under webs or beams or wash them away with the bucket, or just manually clear the webs away with shears or a sword, until finally you can reach the spawner and take a pickaxe to it. Then kill any remaining spiders, clear out the rest of the webs, and check for more lairs nearby.

If you don't care about collecting the webs or supports, one very safe technique is to place a lava source block and build a cobblestone dam just behind it. Stand on the dam, clear out cobwebs as far as you can reach (lava doesn't seem to burn them), then pick up the lava and place it further in. The lava is guaranteed to kill the spiders, and the light keeps them from spawning near you. You also get a chance to place torches as you go. With patience and a bit of care, it is possible to lock up a spawner this way without ever risking a spider bite.

Trivia

 * Before 12w05b, it was possible to place and ride minecarts on floating rails. This can still be accomplished with the use of mods.
 * Entrances to mine shafts can appear underwater.
 * Sometimes, the mineshaft may have a caved-in roof if spawned high enough.
 * Sometimes torches will emit light without being attached to a block.
 * Before 1.4, chests could generate right next to each other, creating a naturally-occurring large chest.
 * At bedrock layer, Abandoned Mineshafts will cut through the top bedrock layer(s). It is unknown if it can cut through the last layer.
 * Due to how deep the shafts can go, it is possible to find exposed veins of diamond ore and emerald ore in corridors.
 * It is possible to reliably generate mineshafts on the surface due to 1.4 customization options in the Superflat world generation menu. One merely has to use the "Tunneler's Dream" preset and remove the layer of stone. This will also cause Strongholds, etc. to generate above ground, often floating, in an odd, disjointed manner.
 * Before 1.4, chests could sometimes be found floating in air.
 * Mine shafts can generate on top of other mine shafts.
 * Rails can float in the air when they pass over a block.
 * As of 1.5.2 Minecarts with chests may be generated on cobwebs.
 * Mineshafts are less common near the origin of the world (0,0).
 * It is possible to find high level enchanted books at lava levels in Minecarts with Chests.
 * In LEGO Minecraft Micro World- The Village, an Abandoned Mine Shaft is beneath the Village.