From Halopedia, the Halo wiki
Montero is a region within the Campos Wilderness District on the human Outer Colony of Gao. A large portion of this territory is covered by a dense jungle filled with native wildlife. Beneath the jungle was a vast cave system that spanned roughly one thousand square kilometers. Unbeknownst to humanity until 2553, the cave system was home to the Forerunner Covert Support Base 4276 and its attached Archeon-class ancilla, Intrepid Eye.
During Operation: JOVIAN WHISTLE, a portion of the Montero Jungle, the cave system, and the Forerunner facility would be destroyed by Blue Team with a HAVOK tactical nuclear weapon in order to prevent the Forerunner technology from being acquired by the Gao Republic.
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