Mineshaft

Abandoned mineshafts are generated structures that generate in the Overworld.

Generation
Abandoned mineshafts generate naturally in the Overworld, they are usually found underground but some can be found in the ocean. They are generated before chunk features such as ores. As with other features, multiple abandoned mineshafts can generate near each other and intersect, producing larger complexes. They can also intersect with other cave systems. Corridors will generate a floor of wooden planks if they cross over empty space.

They can also spawn on the surface in mesa biomes. These are composed of dark oak planks and fences, rather than oak. Cave spider spawners and various wooden pieces will not generate if above ground (i.e. the sky light where they generate is 8 or more), and generated railways will be less complete above ground.

Structure
Abandoned mineshafts 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, although, sometimes it generates in a way for up to five entrances.)
 * Corridors - 3×3 block corridors featuring supports made from oak 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.

Trivia

 * At bedrock layer, abandoned mineshafts will cut through the top bedrock layer(s).[Is this an issue now?]
 * Mineshafts can generate on top of other mineshafts. If you see two different ones intersecting each other anomalously, they are two different shafts, even if they somehow meet up perfectly elsewhere.
 * Mineshafts are less common near the origin of the world (0,0).
 * Mushrooms can be found at the bottom of mineshafts.
 * Sometimes Mineshafts spawn without chest minecarts at all.

Publicity

 * In LEGO Minecraft Micro World- The Village, an abandoned mineshaft is beneath the village.