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, players have a pickaxe (and eventual Drill and Steampunch upgrades) and are tasked with exploring the mines below the town, bringing back the riches that they find and selling them.
There are a couple of mechanics that limit how long Rusty can explore, though. First is light - it is a mine, and as his lantern runs out of fuel, it slowly gets darker in the cave until it extinguishes. The second is the inventory. The gems Rusty picks up all take up a certain amount of pockets, and if all of the (initially 3) pockets are filled, then Rusty can't carry anymore, and has to run back up to sell (or discard something less valuable to make room for something that is more).
The amount of money that players earn from the gems serves as a sort of leveling system. When they first start, only some basic items are available, but as they make more money, players start to unlock more abilities, including upgrading the previous paragraph's inventory space and the lamp. This is also where players power up Rusty's axe. The map is divided to show players when the rocks require a stronger type of axe via a dotted line with a small pickaxe and a number next to it lets them know at a glance when they're hitting somewhere with a new strength requirement. Rusty finds a second type of currency as he digs, small blue orbs that can also be spent in town on upgrades.
The way Rusty dig down into the earth is persistent - he'll be going deeper and deeper, digging a labyrinth of tunnels to get himself 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, players have to make sure that they can hit it while their feet are touching the floor. Players will also want to make it possible to get back up, or else they will get stuck and have to self-destruct and find a new way to approach the level.
In the main cavern, players will find small side caverns that are set up more as puzzle-platform rooms, where they wind up finding some sort of upgrade. This is where the Metroid elements come in - players find the generator to receive an upgrade, such as boots that eliminate fall damage, the drill that lets them get through rocks, or dynamite. These areas also have secrets hidden in them with even more goods to sell back in town.