Old World is the second area of the mine visited by Rusty in SteamWorld Dig, and it's full of humans that were exposed to radiation. Said humans are known as Shiners, who used to live on the surface a long time ago.
Old World adds new elements to the game, such as acid, which can either drop from the ceiling or form pools that hurt Rusty when he comes into contact with it. There are new enemies called Shiners that deal more damage than previous enemies (and occasionally will hold dynamite that they will throw). There is also the addition of Booze Barrels, which will explode after being hit, falling, or by Rusty or an enemy being in close proximity for too long.
There are two types of Shiners. The aforementioned type, and Boozers. Boozers throw booze bottles, which cause moderate damage to Rusty. Boozers are also able to take a lot of damage, however, Boozers are weak to fall damage, so much so that it will always cause instantaneous death.
Like Archaea, there are three upgrade caves to be found as well as other small caves, and a Teleporter about midway.
In the third upgrade cave where the Fall Dampeners are found, and a generator for Vectron. Destroying it will complete half of the achievement 'Sequence Breaker', which requires the player to destroy two generators before entering Vectron.
The upgrades found in the three main caves of Old World are:
- Dynamite - Dynamite can now be used and the player starts with five uses.
- Steam Punch - Another mining tool, that uses water and charges up to destroy blocks.
- Fall Dampeners - Stops the player from taking fall damage.
- In the Steam version of the game, an easter egg referencing Half-Life 3 can be found at the top of the second upgrade cave (pictured to the right). It references the fact that the game still hasn't come out by showing it close to release in the alternate future of the series.
- The Wii U version of the game also has an easter egg, be it much larger. It can be found in the first upgrade cave by Steam Punching upwards, to the left of the entrance. Once the block is broken, a secret path will be revealed and lead to a Mario castle. Rusty can go inside and go through a small area based on level 1-2 from the original Super Mario Bros.. The area features sellable Mario Coins that play the famous sound effect when collected, as well as Mario-themed blocks.
- There's one more easter egg, that can only be found on the Xbox One version of the game. It can be found in the third upgrade cave, in Old World, underneath the area covered in mushrooms. A stick of dynamite is required to destroy the mushroom on top of the entrance to the secret path, which leads down to a small area containing a Halo Warthog that can be flipped in a similar fashion to the Halo games. Once flipped three times, a door will open to reveal a big orb. A base from the famous Bloodgulch map can also be seen in the background.