Mineshaft

Mineshafts (also known as abandoned mineshafts) are generated structures that generate underground in the Overworld.

Generation
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 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 generate on the surface in badlands biomes. These mineshafts 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 more complete above ground. Mesa mineshaft also usually have more ore. The most common types of ore are gold and coal.

If you are looking for ores, mineshafts are probably the best place to look because lots of ore generates there, and there's lots of lava for generating diamonds.

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 or dark oak fences and wood planks (sometimes complementary with torches), broken rails, minecarts with chests (containing resources) and cave spider spawners surrounded by cobwebs. On long corridors, supports are placed four blocks away from each other, and some have a missing plank on the middle. Underground tunnels can have imperfections on the upper side, sometimes occupying the area of the missing plank.
 * Crossings - intersections with wooden pillars, sometimes two floors tall. They occupy a 5×5 spacing.
 * Staircases - diagonal sections of tunnel.

Trivia

 * When generated, abandoned mineshafts will cut through the top bedrock layer(s), if they happen to generate at the bedrock layers.
 * 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.
 * Mushrooms can be found at the bottom of mineshafts.

Publicity

 * In LEGO Minecraft Micro World – "The Village", an abandoned mineshaft is beneath a village.