Tunnelers were reptilian creatures of Po-Metru capable of taking on the properties of any physical object used against it (for example, a fireball would turn it into a creature of fire).[1, 2] Tunnelers lived and appeared periodically in Po-Wahi.[3, 1]
Tunnelers had been a problem in Po-Metru for as long as Onewa could remember. They were lizard-like Rahi, normally about twice as long as Matoran were tall, with an appetite for solid protodermis. They had been known to dig up into warehouses and consume everything from raw protodermis blocks to finished tools. They rarely posed a real threat to the Matoran workers, but every now and then one went bad and began rampaging through the work areas. Rogue tunnelers were known to cause Po-Matoran crafters to run away en masse. The one Onewa saw in the Sculpture Fields was bigger than any he had ever seen, easily three times the size of a Toa. Worse, its scales were mottled with dark patches, and its eyes were red. In a tunneler, both were sure signs of madness. It walked on four legs. Tunnelers absorbed whatever power was thrown at them. For example, if a tunneler was hit with a fireball, it would transform into a creature of flame; if it was hit with a rock, it would turn into a creature of stone. If it came into contact with glass, its body would become prone to cracking, effectively preventing it from moving and disabling it.
Tunnelers resembled great lizards. As their name suggested, they lived underground and used their claws to tunnel through solid stone. They defended themselves by changing their body mass to match any material which they came into contact with. For example, a tunneler struck by a fireball would become a creature of fire. The only way to defeat a tunneler was to bring it into contact somehow with a substance that would leave it with a weaker form, such as glass or water.