From Halopedia, the Halo wiki
Mato Grosso is a province on the humany colony of Mamore.
In 2511, the province was rife with terrorist activity. One such group, the Freedom and Liberation Party, detonated a nuclear device in the Haven arcology in 2511, killing two million and injured over eight million more.
The province was likely named after the Brazilian state of Mato Grosso, literally meaning "thick forest".