Earth

Summary
Earth is the home planet of Humans, it consists of seven continents. It is home to a wide array of different creatures. Earth's history goes back as far as about 4.6 billion years ago when the planet first formed, with the first single celled organisms appeared about 3.5 billion years ago. These simple organisms eventually began to evolve into more and more advanced creatures. A certain species of exceptional intelligence, humans or Homo Sapiens, evolved about 200,000 years ago. They eventually began to show signs of sentience and made it into space in 1961. Several centuries after they became a spacefaring race, humans encountered a collective of intelligent, hostile extraterrestrials known as the Covenant. Thus begins the story of Halo.

Discovery of Earth
The Covenant originally had no knowledge about humanity's home world, mostly thanks to the development of the Cole Protocol. Sometime between the events of Halo and Halo 2, the Covenant eventually discovered Earth's location. The Prophet of Regret led a small invasion force, but was surprised to find out that it was the humans' homeworld. At the end of Halo 2, the Covenant had eventually began a full-scale invasion of Earth, and have completely overrun Africa by the beginning of Halo 3.

Many theories about Earth erupted when the Halo 3 Announcement Trailer was shown.

The Covenant, had discovered a massive Forerunner structure several miles wide buried under the ground where the city of New Mombasa used to be. Is this the Ark? If so, why was it on Earth?

The Prophet of Regret had obviously come to Earth in search of a Forerunner artifact that might determine the fate of all sentient life in the face of the galaxy, as the massive firepower that the Halos yield could destroy all sentient life "within 3 radii of the galactic center," as stated by a Monitor in the games.

Africa is the most important place on Earth in regards to the Halo story, it seems, and, coincidentially, it is theorized to be the birthplace of humanity.

It is revealed through the storyline of the first two games that the Forerunners have some sort of connection to humanity.

Earth locations
Africa
 * New Mombasa - Kenya
 * New Mombasa Orbital Elevator
 * Old Mombasa - Kenya
 * Diego Garcia - Island
 * Zanzibar Island - Island

Austrailia
 * HighCom Facility Bravo-6 - Sydney
 * Sydney Synthetic Intellect Institute - Sydney

Asia
 * Songnam - Korean Peninsula

North America
 * Chawla Base - Boston
 * Havana - Cuban Island
 * Centennial Orbital Elevator

Luna
 * Luna OCS Academy