From Halopedia, the Halo wiki
The Yucatán Peninsula is a peninsula on the southeastern edge of Mexico on Earth. The island of Cozumel sits off the eastern coast of the peninsula.
During the Covenant occupation of Earth in 2552, they conducted extensive underwater excavation operations in the area, looking for Forerunner artifacts. Blue Team, comprised of Spartans Frederic-104, Linda-058 and Will-043, stopped these operations.