The Terra-Firmians are a race of sentient underground creatures.
Divided between the Terries (who wear bow ties) and Fermies (who wear neckties), the Terra-Firmians are a people of humanoid multicolored creatures who have the ability to roll themselves up into balls to resemble rocks. The Terra-Firmians' origins are shrouded in mystery (though they may be related to Arendelle Trolls), but they live in a gigantic network of caverns beneath the American continent, which they call Terra-Fermy. Most of the Terra-Firmians living beneath Duckburg speak in a cowboy-like dialect from listening to western radio shows through the radio cables that run through Terra-Fermy. The Terra-Firmians are led by a King.
Every year, the Terra-Fermians participate in an annual sporting event called the Earthquake Games. The goal of the games is to see which team can cause the larger earthquake. The Terries cause earthquakes by rolling against giant pillars supporting the Earth's crust, while the Fermies gather together under the crust and lift it. A trophy (usually a token from aboveground) is offered to the winners.
Scrooge attempted to stop their earthquakes, for fear that his Money Bin would be damaged by them, by stealing their trophy, an ancient vase that fell into Terra-Fermy during an earthquake they caused once. They caused a large earthquake to prevent the ducks from escaping. They did escape (though Scrooge had to abandon his top hat), but all of the money from Scrooge's bin fell into Terra-Fermy. However, through listening to radio contests, the Terra-Firmians had become convinced that money was worthless, and sent it back up. They then kept the top hat that Scrooge had dropped as their new trophy.
Some time later, the Terra-Firmians were met by Scrooge once again, this time joined by Gyro Gearloose, Bentina Beakley, Webby Vanderquack, and his butler Duckworth. Later on still, the Terra-Firmians had to deal with dangerous detritus dumped in Terra-Fermy by corrupt aboveground businessman Flintheart Glomgold, from which Scrooge McDuck rescued them.