Deeper Underground introduces a fully new campaign set in the Roaring Twenties, complete with a new town, new buildings, a new mayor, and balloon trips to special deep-drilling grounds. Two new playable characters, Thorpe and Mabel, join the roster, alongside new vendors Yvonne, Sanjay, and Dynamite Dale, while returning NPCs Edward, Raynor, Fred, and William continue supplying drilling equipment by train. Philip also returns with a new side game called Dice Dice Dice.
Alongside oil, players can now discover water underground, which speeds up drilling and reduces costs, and serves as an essential ingredient for producing petrol. Petrol cannot be sold directly; it must first be processed in refineries, adding a production layer to the resource chain. Dynamite is also added as a tool for breaking through hard rock that standard drills cannot handle.
Every few in-game years, standard drilling levels alternate with special expedition missions focused on drilling as deep as possible with limited resources, pitting players against opponents in a depth challenge. Town shares remain the win condition, using the same bidding system from the multiplayer campaign, with additional shares available through deep-drilling mission performance.
