Underground Structures

Caves

These are natural caverns, usually consisting of long, winding passages and caverns, often containing underground streams and lakes. As you go deeper, you will increasingly run into pools of lava, too.

Dungeons

These structures contain a [mob spawner](mobs.html#mob-spawners, and 1 or 2 chests full of treasure! You will have to battle skeletons, zombies or spiders to get the goodies, though.

The green mossy cobblestone is a sure sign of a dungeon.

Inactivate the mob spawner by placing torches all around it, or destroy it with a pickaxe.

<p>Placing torches like this will make the dungeon safe.</p></div>

Abandoned Mineshafts

You’re not the first person to mine these lands after all! You will often come across somebody else’s abandoned mineshafts.

They contain rails, wooden supports, treasure chests, and spawners producing poisonous cave-spiders.

The spider-webs can be cleared with a sword or shears to provide string (and string can be converted to wool). The supports provide a good underground source of wooden planks and fence posts (which can be burned for fuel).

Ravines

These huge chasms are spectacular sights, but don’t fall off a ledge!

Strongholds

These underground fortresses are built from a different kind of stone brick, and contain doors and railings that are not found elsewhere. You will also find mystical libraries, and a special mob called a Silverfish that isn’t found anywhere else.

The final thing you will find is a chamber containing an End Portal, and that’s your main reason for wanting to find a Stronghold.

Exploring Underground Structures

Exploring underground is a fun part of the game; it can also be a great way to find precious ores and minerals.

There are three main dangers when exploring underground; monsters, falling into lava, and getting lost.

Source: http://www.minecraft101.net