The Great Mask of X-Ray Vision
Allows the user to see through walls and discover that which is hidden.
Source: BIONICLE #2: Deep Into Darkness
The Mask of X-Ray Vision, which allowed the wearer to see through solid objects. Many Akaku were fitted with special lenses that allowed for telescopic vision as well.
AKAKU NUVA (ah-KAH-koo NOO-vah)
The more powerful Mask of X-Ray Vision worn by KOPAKA NUVA. He used this mask to spot the damage done to KO-KORO by the RAHKSHI. Like all KANOHI NUVA masks, its power could be shared by the user with those in close proximity. See KANOHI NUVA.
Source: BIONICLE Encyclopedia Updated