Koros is a planet in the Solana Galaxy featured in Up Your Arsenal. It is a dark, cold planet covered in oceans with civilization built in cities across its islands. The planet is the site of the Nefarious BFG, a large fortress within one of the cities, used as the recharging station for the Biobliterator.
Ratchet and Clank traveled to Koros in order to destroy the first Biobliterator. In order to do this, they found a large ion cannon in the Nefarious BFG, and took it over to use it against the Biobliterator and destroy it.
Koros was at some point occupied by Dr. Nefarious, as seen with several of the destroyed bridges around the city, which appears uninhabited. After the Biobliterator transformed the population of Metropolis into robots, it refueled on Koros, before it would attack Veldin next.
After Al decoded the master plan, he learned that the Biobliterator was recharging on Koros. Sasha Phyronix then sent Ratchet and Clank there to destroy it. When they landed, Ratchet lamented he would be unable to damage the Biobliterator as even his own weaponry would deal no damage. Clank then noticed a large cannon on the planet within the command center, and suggested he and Ratchet take control of it.
In "Get inside command center", Ratchet then fought through Nefarious' robot garrison of drone bots, mega bots, and soldier bots to reach it. Once inside, during "Access big gun control panel", Ratchet defeated another garrison within the command center and got Clank inside the control panel room. Clank then destroyed the Biobliterator. As he did, Sasha contacted them to inform them that a second Biobliterator had been constructed on Mylon, but the fight would be more difficult as this time Nefarious would be prepared.
Koros is a cold, dark planet with black waters and small islands across its oceans, comprised of rocks, on which there is a little green vegetation. From space, it appears to have small continents also made up mostly of rocks, and is otherwise mostly covered in water. It also appears to have thick fog. Three astronomical bodies are seen in Koros' sky. The planet is also home to sharkigators. The Nefarious BFG area is a city built on one of the islands.
At the heart of the city, in contrast to the surrounding areas, is a black, metal fortress that is the command center of the Nefarious BFG, with an immense ion cannon atop the fortress, and the refueling station for the Biobliterator. Around the central fortress, several flying ships are seen, including tyhrranoid dropships.
The Star Explorer lands on an outer portion of the city area.
The city is made of mostly stone buildings in between its walls, spanning across islands connected by long stone bridges. Around the buildings are tables with beach umbrellas set up, and flag streamers hanging across the streets, suggesting it to be a tourist town, but is now entirely occupied by Nefarious' forces. Many buildings also have glass domes or balconies, and some parts of the city are walled off by metal gates. The city is brightly lit by street lamps, as well as glowing antennae atop the buildings.
The city area surrounds the Nefarious BFG, and contains a path to the command center, seen in "Get inside command center". The path begins along the outside of the city, with a few destroyed bridges that previously led to an adjacent island, and a few buildings and tables outside the walls of the main city. A road then leads inside to the main city, past a small grassy plaza on the left of the road surrounded by an aqueduct, and into a wider plaza around several houses.
The road then bends right to a bridge across to another island in the city, with a tower halfway across the bridge, spanning the road. Across the bridge is a small plaza with a few houses, and then continues through a locked gate, before bending around to a set of houses atop the ledge. This block of houses can also be accessed through using a stack of metal boxes beneath the ledge to the right of the plaza. The plaza is a wide area with a few large houses, more tables. It is located directly in front of the entrance to the command center, which is a tall open door leading inside.
The command center of the Nefarious BFG is a large fortress visited in "Access big gun control panel". The center is a vast, circular room with several walled-off portions based around the control panel room in the center. The room has a transparent roof, and around the edge is a corridor reached with a ramp in a room filled with computer terminals. This corridor leads to a bridge into the control panel, a small round with several computer terminals and large cables around a seat that controls the big gun.