From Halopedia, the Halo wiki
United Republic of North America, Earth
Base Segundo Terra is a UNSC military base in the United Republic of North America on Earth, specifically located near Mexico City.
During the Battle of Earth, the Spartan Blue Team used the base as a staging point to attack the Covenant incursions in Havana and the Yucatan Peninsula.
"Segundo Terra" roughly translates from Spanish, with some grammatical caveats, as "second earth" in English.