From Halopedia, the Halo wiki
The plasma drill, also called a mining laser, is the sole armament of the Harvesters used by the post-war Covenant. It is primarily used as an excavation tool for the purposes of recovering Forerunner artifacts, although it is also utilized as a weapon.
The plasma drill fires a single greenish-yellow energy beam, similar to that of a focus cannon. At full power, the beam is capable of drilling through multiple layers of solid rock.
During the Requiem Campaign in 2558, Fireteam Crimson used a captured Harvester's plasma drill to bore an entrance through a rock wall into the caverns where a hidden Forerunner structure contained one of the artifacts trapping the UNSC Infinity on Requiem.
During the Battle of Aktis IV, one of Jul 'Mdama's Harvesters used its plasma drill to destroy the caves where Sali 'Nyon and his followers were hiding. Another Harvester ambushed Sarah Palmer and nearly killed her with a blast from its plasma drill. Although Palmer survived, she was trapped in a crevice and unable to stop Catherine Halsey from escaping with the second half of the Janus Key.
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