From Halopedia, the Halo wiki
Mare Erythraeum is a large, dusky region on Mars that is visible from Earth with a simple telescope. The name comes from the Latin for the Arabian Sea, because it was originally thought to be a large sea of liquid water. It was included in Percival Lowell's 1895 map of Mars.
In late 2552, Mare Erythraeum was the site of the a battle in which UNSC Army forces led by Colonel James Ackerson fought ground forces from the Covenant's Fleet of Furious Redemption. The UNSC forces were defeated, resulting in the loss of the UNSC's Mars garrison and a subsequent increase in Covenant force deployments on Earth. Colonel Ackerson was taken prisoner and later interrogated by the Jiralhanae Shipmaster Lepidus.
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