Minecraft (Pocket Edition)
The above image is of an Abandoned Mineshaft with Rails.
Location / Appearance
Abandoned Mineshafts are a relatively common multi-room, multi-floor structure you will encounter when caving. They are distinguished by their wooden support structures and bulkheads (as seen in the above image). They also often have flooring made from Wooden Planks and frequently have sections of Rail.
Usage / Purpose
The best part of Abandoned Mineshafts, besides the fun of exploring, is the amount of Rail that can be collected. Dump a Water Bucket over a section of Rail to make it float up for easy collection. The spilled water can be tapped on with the empty Bucket selected to place it back in your bucket for reuse. If you lack a Water Bucket you can mine the Rail as you would any block and it will yield one unit of Rail.
Abandoned Mineshafts are fun to explore, but can be dangerous. Watch out for sections of missing floor.
Be sure to mine through the ends of passages. Sometimes you find connecting sections of Mineshaft that have been closed off.