The enemies in this level are these worm things, they're a simple enemy and they just take one hit to kill.
These bats are the other enemy, they swoop in an arc and when they are directly above you they drop down on your head, they're a very annoying enemy but only take 1 hit.
You head to the right so you can go down a lift and mine some treasure, the lift is stuck though and needs a key so continue on to the right.
There you'll find a giant treasure chest with the key, after collecting it go back to the left where the elevator is.
After going down the elevator head through the false wall on the left to get your third health upgrade, after collecting it continue down the shaft.
Head to the left and a boulder drops behind you and chases you, run to the left as fast as possible.
Head up the chain and follow it around to the left, eventually another boulder chases you, run to the right and the boulder clears the path for you to continue.
Head all the way to the right jumping on the heads of fish and climb down the ladder.
Head to the left and another boulder follows you, after continuing all the way to the left it clears the path for you again.
Another mine cart section! You know what to do, jump and duck and switch mine carts until you break through the rocks.
Now you're approaching the centre of the earth. Continue to the left and head down the ladder.
Head to the right and over the fish and down the ladder.
Follow the path down and to the left and jump on this stone here to drop into the boss arena.
The boss is the Terra Firmie King, I have no idea what he's supposed to be though but he challenges you to a game.
His first attack is rolling straight at you, note that you can't hit him at all while he's rolling, only when he's stopped.
His second attacks is charging up for half a second and then doing 3 laps around the wall of the arena, he then jumps back and performs one of two attacks.
His first attack is just dropping boulders on your head, easy to dodge if you keep an eye on the shadows.
His other attack is rolling balls at you, this attack can be pretty tricky to avoid taking damage on.
After performing either of these attacks he jumps back and lets you hit him, after three waves he concedes and gives you the treasure you were after. The Giant Diamond of the Inner Earth.