In the coal mine, there is coal, obviously, and cobblestone. There are also Crypt Ghouls there. They are quite powerful.
There is gold, there is iron, there is cobblestone. Also, Rusty will apply telekinesis to any item for 100 coins. Talk to the Lazy Miner, and find his pickaxe. It can be found at -18, 24, -305, and can only be picked up during his quest.
The Deep Carverns have many levels, and I will go through them 1-by-1. Note that the lift operator can only take you to each level after you have discovered it.
Here there is gold and iron, and creepers.(Why do you think its called GUNPOWDER mines????)
You can mine Lapis here, but from this level onwards you will need good armor and a sword as the mobs are VERY powerful. The zombies in this level will drop: Rotten Flesh, Lapis Armor(Full set gives you extra 60HP), and Lapis Crystals, a reforge stone.
Here, you can mine redstone, and there are pigmen. From here onwards, it gets quite confusing to find the next level.
This is where you mine emeralds as well. The slimes here deal a lot of damage but are easy to kill. My favourite method of killing them is getting a bow with at least 100 damage, then enchant it with: Aiming 5, Piercing 1, Power 5 and Telekinesis. Spam arrows to kill them!
The mobs here are zombies and skeletons, with diamond armor and weapons. The ones here drop a piece of Miner's Armor, which gives you Regenaration 8 seconds out of combat if you have the full set. The ones in the next level, though, are more powerful and they drop the same set of armor, but it comes with Protection 5
Here, you can get obsidian, diamonds and diamond blocks. The diamonds and diamond block are found everywhere, but the obsidian is generally found around the lava. The mobs here are the same as the ones in the diamond reserve, but their items and armor are enchanted.