Image & Form's SteamWorld Dig is the second game in the SteamWorld universe after 2010's SteamWorld Tower Defense on DSiWare. Players dig their way through randomized worlds to uncover treasures and thwart the evil that lurks below to help get the mining town of Tumbleton back on its feet.
Probably the best way to describe Steamworld Dig is as a sort of Metroid meets Mr. Driller game. As Rusty, you have your pickaxe (and eventual Drill and Steampunch upgrades) and you're tasked with exploring the mines below the town, bringing back the riches that you find and selling them.
There are a couple of mechanics that limit how long you can explore, though. First is light- it IS a mine, and as your lantern runs out of fuel, it slowly gets darker in the cave until it extinguishes. The second is your inventory. The gems you pick up all take up a certain amount of a pocket, and if all of your (initially 3) pockets are filled, then you can't carry anymore, and you have to run back up to sell what you have (or discard something less valuable to make room for something that is more).
The amount of money that you earn from the gems serves as a sort of levelling system. When you first start, only some basic items are available, but as you make more money, you start to get more things that you can do, including upgrading the previous paragraph's inventory space and the lamp. This is also where you power up your axe- as you dig further into the mines, you'll notice that the pickaxe isn't moving through blocks as quickly, and you have to upgrade in order to go faster. The map is actually handily divided to show you when these divisions occur- a dotted line with a small pickaxe and a number next to it lets you know at a glance when you're hitting somewhere with a new strength.
You find a second type of currency as you dig, small blue orbs that can also be spent in town on upgrades.
The way you dig down into the earth is persistent- you'll be going deeper and deeper, digging a labyrinth of tunnels to get yourself further down and find new treasures. Rusty can't use his tools when he's in the air, so in order to dig something up, you have to make sure that you can hit it while your feet are touching the floor. As you progress down you want to be smart with your digging, since you might actually make it impossible to reach a treasure again (until the eventual acquisition of the incredibly useful Steampunch, that is). You also want to make it possible to get back up, or else you'll get stuck and have to self-destruct and find a new way to approach the level.
In the main cavern, you'll also find small side caverns that are set up more as puzzle-platform rooms, where you charge tasks and eventually wind up finding some sort of upgrade. This is where the Metroid elements come in- you find the glowing generator looking thing, stand on it, and are blessed with an upgrade, such as boots that eliminate fall damage, the drill that lets you get through rocks, or dynamite. These areas also have secrets hidden in them with even more goods to sell back in town, so be sure to look around!