Abandoned Mineshaft

Abandoned Mineshafts are naturally generated structures that form underground, that were added in Update 0.9.0.

Structure
Abandoned mineshafts are long winding maze-like mines. Mineshafts begin with a very large dirt room, with an arched roof, and then they continue into a long tunnel-type maze, with supports and crossbars (fences & wooden planks) approximately every 4 blocks. These mineshafts are made of oak wood.

Occasionally these can have torches spawning or have chests inside filled with loot. Very commonly, these will have rails along the floor, and many Cobwebs as well. Most mineshafts contain Cave Spider Mob Spawners as well but some will generate without them.

Mesa Mineshafts
Main Article: Mesa Mineshafts

Mesa Mineshafts are Abandoned Mineshafts that spawn in Mesa Biomes. They are made of Dark Oak Wood rather than Oak Wood and spawn on the surface. Gold Ore is found in plentiful quantities in these mineshafts.

Trivia

 * Before Update 0.11.0, Mineshafts contained Spider Spawners.
 * Mineshafts can occasionally be intersected by a Stronghold or a Dungeon.
 * Some areas of the Mineshaft are lit (thought its not enough to stop mob spawning)
 * Sometimes part of the shaft can be missing or on fire due to a natural lava fall.
 * Mineshafts are not only long in length, but can go from sea level to bedrock.
 * Mineshafts are more likely to spawn the deeper underground you go.