The Arcadia Forerunner ruins are a series of Forerunner ruins in the jungles of the planet Arcadia and were the site of a conflict between United Nations Space Command and Covenant forces in the year 2531. Not much is known about the possible functions or purposes of the structures.
The ruins are located in a basin, overlooked by a plateau where the ONI Base resides. The ruins consist of numerous temple-like structures, and a main structure, which is likely an entrance to some sort of underground complex. The temple structures radiate from the main structure in a half-circle shaped pattern. In some areas, Forerunner floor texture can also be seen under the ground dirt.
- Main article: First Battle of Arcadia
In 2531, the Covenant were directed to Arcadia, by a structure on Harvest that pointed them towards the planet. The Covenant forces traveled to Arcadia with two CCS-class battlecruisers. One was destroyed, but the other engaged in surface activity, attacking Pirth City and searching for Forerunner artifacts on the planet. After they found the ruins, the Covenant deployed a massive dome-shaped energy shield on the planet surface to prevent the UNSC forces from reaching the structures. While scouring the ruins, the Covenant found the coordinates of a Forerunner Shield World. After receiving the information, the Arbiter Ripa 'Moramee and the Prophet of Regret immediately traveled to the newly discovered installation. On the Shield World, the Covenant quickly realized that they could not unlock some of the valuable artifacts contained within. Regret ordered the Arbiter to capture professor Anders, as she could possibly be used to activate the installation.
On Arcadia, the UNSC forces established a base at the abandoned ONI Base in proximity of the ruin complex, and managed to breach the energy barrier with a few Rhinoes, and after securing the area they established a base and proceeded to investigate the area. However, the Covenant planned to keep the UNSC forces away from the main structure by building a Super Scarab, which they were upgrading with technology obtained from the ruins.
The Covenant tried to destroy the UNSC base, by using the Scarab's main cannon to destroy the temple structures protecting it. However, the UNSC forces managed to weaken the Scarab by destroying its power nodes, and eventually the Scarab itself. When the Scarab had been destroyed, Ellen Anders proceeded to investigate the area, under the watchful eye of Sergeant Forge. While Forge and Anders were investigating the ruins, the Arbiter appeared, and captured Anders. Shortly thereafter, the Covenant ship jumped out of the system, and the UNSC Spirit of Fire followed.