From Halopedia, the Halo wiki
COALMINER is the alias of an Office of Naval Intelligence operative involved in highly classified missions, namely Operation: HYPODERMIC. Given their rather frequent correspondence, COALMINER's handler may be Codename: SURGEON.
COALMINER's earliest known activity was in 2532. In a message to Codename: SURGEON concerning the SPARTAN-III Program, COALMINER noted the early stages of preparing Alpha Company were progressing satisfactorily and expressed confidence in Kurt-051's training regimen.
In 2552, COALMINER detected an intrusion into the ONI databases at Reach. While COALMINER failed to identify the intruder, the intrusion threatened to compromise HYPODERMIC. COALMINER informed SURGEON of the breach and deleted the UNSC Circumference from Gamma Station's database to minimize the damage.
This caused difficulties during the latter stages of the Fall of Reach, as it prevented the AI Doppler from erasing the Circumference's NAV database in accordance with the Cole Protocol. A SPARTAN-II team was deployed to destroy the ship and suffered casualties in the process. In an indirect manner, this lapse in security inadvertently saved humanity: in the wake of the mission to Gamma Station, John-117 traveled to Halo Installation 04 aboard the UNSC Pillar of Autumn, which set about the final stages of the Human-Covenant War.
At some point in the post-war era, COALMINER received a series of five reports from SURGEON discussing the state of the post-war galaxy.
List of appearances