Background story:
Link to More Info: http://halo.wikia.com/wiki/New_Mombasa
On October 20th, 2552, a small cluster of Covenant warships exited Slipspace just off of Earth's defensive network. During the ensuing battle, a Covenant Assault Carrier breached the orbital fortifications and made its way to the surface. The ship, which carried the Covenant Hierarch known as the High Prophet of Regret, took position over New Mombasa, deploying troops and vehicles which occupied the city. The city's civilians either evacuated, took shelter, or were killed. In Old Mombasa, announcements in Swahili are broadcast over a public address system. These announcements encourage New Mombasa's citizens to fight against the polytheistic "alien zealots."[2]
The Master Chief arrived in Old Mombasa to help UNSC forces liberate the city from the Covenant. After fighting through the maze-like streets of the mainland, the Master Chief crossed the empty beach below the seawall and then entered the highway system and the bridge to New Mombasa. After a series of quick battles in a public park (the Amphitheater) and the city streets, the Chief boarded and destroyed a Covenant Scarab as it passed through the city's artificial canals. The High Prophet of Regret fled from Earth in his flagship, with the Chief and the In Amber Clad pursuing. The Slipspace rift that the ship opened filled the whole city, but didn't destroy it all, just causing heavy damage to most structures. Although the whole of Mombasa Island was consumed by the Explosion, only the buildings close to the Slip-space vortex were sucked through, though two of its three bridges linking it with the mainland were destroyed.[3] What remains of the city is used by all of the UNSC forces still stationed within the city.
[edit] Aftermath
The Tanaga district of New Mombasa after Regret's Assault Carrier jumped inside of the city. Note the wreckage of the Space Elevator towards the left in the distance.
Originally, it was assumed that the slipspace jump of the Prophet of Regret's Assault Carrier had completely obliterated the city, creating the crater seen later in Halo 3. In fact the city, while devastated, still stood long after the ship jumped, and UNSC and Covenant forces clashed for control of the region. It is assumed that the area was later excavated during the Second Battle of Earth, either by Type-47 Ultra Heavy Assault Platform's or by Covenant warships, revealing the Portal to the Ark. Although it was never destroyed.
During the battles for control of the city, an ODST force was inserted to bolster the ground forces already present - the majority were caught in the blast caused by the Assault Carrier's transition. It is unknown exactly how many ODSTs were killed on their way down, though at least six survived landing.[4]