From Halopedia, the Halo wiki
The Sundark Sea is a body of water on Alluvion, a human colony planet. The sea is home to both qothal and paelosur, aquatic species that have been found on other worlds once under Forerunner authority.
At some point, the Forerunners established a series of shield pylons in the sea that surrounded a depository of an exotic fuel source. However, millennia later, the pylons were deactivated and had gone unnoticed when the Unified Earth Government colonized Alluvion.
In 2554, when the UEG returned to Alluvion to recolonize the planet after its fall in 2542, the recolonization support fleet discovered the now-activated pylons in Sundark Sea that forced back massive walls of water and revealed the seafloor. After several years of surveying this area, an exploration team discovered the exotic fuel source and estimated that it was capable of replacing deuterium for human starships. Contacted by the United Nations Space Command, the Liang-Dortmund Corporation established the Alluvion Research Center within the sea in 2556, seeking to directly power a number of prototype shuttles with this newfound energy source. However, a year into their research, Liang-Dortmund's mining efforts mysteriously triggered the complete withdrawal of the shield pylons and the Sundark Sea collapsed on all who remained within the ARC.
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