Anodyne Spirit

The Forerunner Dreadnaught was used by the Covenant to power all of High Charity. When the Flood attacked the city, the Prophet of Truth uncoupled the ship from High Charity's power grid and used it to travel to Earth. The Dreadnaught travels vertically, like a rocket. It is also capable of Slipspace travel.

A ship with enough power to power all of High Charity could likely move faster than any other ship in existence, which is quite impressive for a 100,000+ year old machine.

At the end of Halo 2, SPARTAN-117 boards the dreadnaught as a stowaway in his mission to kill the High Prophet of Truth. It is possible there is a Forerunner A.I. in the ship, the evidence being the presence Cortana found while trying to slow its launch sequence. . This ship is very geometric and angular in its design, and is estimated to be about 14 kilometers (8.694 Miles) tall. Lord Terrence Hood was meant to destroy the Dreadnaught, but was stopped when he found out that the Master Chief was aboard.

The Dreadnaught landed in Africa, a continent on Earth.

It is possible that the Forerunner ship is an undestroyed Key Ship, which were used to travel to the Ark. This is evidenced by it's apparent activation of the Ark portal on Earth while it was landed in the middle, and Truth calling the Ark portal on Earth it's "true resting place" during gameplay dialogue. However, its presence there during activation may be mere coincidence.