When mining underground, you can sometimes find abandoned mineshafts. Mineshafts are usually made of oak wood, and consist of tunnels underground. In mineshafts you can find rails. These can be mined with anything and used to build rail sytems to get around your world faster. You can also sometimes find regular minecarts and minecarts with chests. open minecarts with chests the same way you would open regular chests. Mineshafts usually have a starting room, wich is usually a small square room with a dirt floor. Mineshafts can be pretty big, so if you get lost in one, you can go back to the starting room to know where you are. Mineshafts also almost always have cave spider spawners. Cave spiders are like normal spiders except they are smaller, more blue in color, and when they attack you, you will get the poison effect. You can disable the spawner by placing torches on all of its sides except the bottom. You can locate cave spider spawners easily because of the masses of cobwebs that spawn around them.
