Abandoned mineshafts are generated randomly with each world, and you typically find them underground.
The easiest way to find a mineshaft is to look in areas where they might be exposed, like ravines. Look for oak wood planks and torchlight to spot them. Also keep an eye out for them when you're traveling in the ocean.
Mineshafts actually generate with a "starting''point. This is a plain squareish room with a curved ceiling, a dirt floor, and exits often on all sides.
Mineshafts are a great source for rails, especially in early game when you may not have much iron. You can also find chests in minecarts that hold rare items. They may hold pumpkin or melon seeds, bread, coal gold, iron, lapis, and redstone.
When you're in a mineshaft, be on the lookout for cave spiders.
Mineshaft are the only places you'll find cave spiders.
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Encyclopedia For Minecrafters
RandomAn A-Z book of tips and tricks the official guide don't teach you Credits to Megan Miller coz I copied her book For those complaining why I copied her book, its because for those who don't have it can read this instead