The Underground Monsters, or Diggers, are a group of humans that were experimented on by Shredder's Foot Technicians, and turned into monsters capable of surviving deep underground. They appear in the 2003 TV series and 2003 video game. At least seven are known and four have official names: Quarry, Stonebiter, King Nail, and Razorfist. The other three are a Underground Spider Monster, Underground Insectoid Monster, and Underground Cyclops Monster. The effects of the Moon Crystal found in the Underground City can revert them back into humans.
The Foot Clan captured a number of innocent people and subjected them to horrendous experiments, transforming them into monsters capable of surviving underground. The Foot used these creatures to search the underground beneath New York for the Utroms and discovered an Y'Lyntian settlement. Once it was determined that the Utroms were not hiding underground, the Diggers were ordered to be terminated. However, some of the Diggers retained their humanity and rebelled against the Foot, who then abandoned their underground outpost. Others were not fortunate, prowling the tunnels, becoming nothing but mindless beasts.
The sentient Diggers, composed of Quarry, Stonebiter and Razorfist, were forced to hide in the old Foot outpost, using a power crystal to generate a force field to keep the others out.
When Michaelangelo incidentally caused two Y'Lyntian crystals to touch, a sound reached into the underground. The Diggers feared the crystals and went looking for the source. After some misunderstanding with the TMNT, the Diggers cooperated with the Turtles to find another crystal to maintain their force field, going deep down into the catacombs where they found the Y'Lyntian city. Pursued by the others, each of the Diggers were soon transported into stasis chambers by the Entity, the city's sole Y'Lyntian guardian, where the crystal moon that powered the crystals could restore them to human form. Though he claimed he wanted the Diggers' aid in repopulating the city, it is presumed he intended them to be the slaves of a new Y'Lyntian empire.
After the TMNT imprison the Entity, they find the Diggers have reverted to human form. Unfortunately, they soon discover that straying too far from the crystal moon causes them to revert into their monster forms, forcing them to stay in the city. As the TMNT promise to return, the Diggers began to exhaust themselves trying to find a permanent cure. After using up a lot of crystals, they tried using explosives to break a few chunks from the crystal moon. Unfortunately, they accidentally dislodged the crystal moon from the cavern roof, causing it to fall into the lava. Without its regenerative effects, the Diggers reverted to their monster forms, attacking anything that moved.
By the TMNT returned with a cure, the Diggers attacked them. Donatello first administered the cure to Quarry, who reverted to her human form, Sydney. But to use more crystals to inoculate the others, they needed to restore the crystal moon. After raising the crystal moon from the lava, the Diggers became human again and were given the cure, enabling them to return to the surface.