Undergrounder is an enemy fought exclusively in the Game Boy Advance and later versions of the game at the Whisperwind Cove between floors 30-40, being a dangerous foe due to its constant usage of the Quake spell.
Tunneler is an enemy fought at the Sealed Cave area, and later as a boss in the Tower of Babil. It uses Earthquake in both battles as well as the Constrict ability, and in the boss fight, will counter all attacks with Earthquake and uses Constrict as its normal attack.
Tunneler is first fought as a boss in Ceodore's tale in the Mist Cave, then later as a normal enemy during the Crystals Tale around Damcyan and Agart. In both battles, it will constantly attack using Earthquake as before.