Ruins of Ultrethqwe

Nobody knows exactly why the Ultrethqwe tower was constructed. The Forerunners never established a significant colony on Ketesh, only using it as a stop-off point for their resupply facility orbiting a gas giant further out in the system. All indications are that the tower was part of a larger complex of structures, but suffered significant damage from an asteroid impact in approximtely 14,000 B.C.E., leaving the tower as the only remaining structure.
Western Kaaran has always regarded the Quenyathar Valley as the epitome of Forerunner settlement of the planet, the underground structure protected by the warrior monks of the Bringers of Holy Light until their persecution by the Covenant and replacement by a Covenant legion. The eastern civilisations of Kaaran looked instead to the tower as the greatest local accomplishment of the ancient race - even after a hundred millenia, the tower not only stands, but is still protected by the solid light barriers it generates. All attempts by the Kaaranese have failed, and the Prophets only ever attempted to gain access to it once, also meeting with failure. Legends hold that only one person has ever entered it successfully - Morheka the Great, a legendary Kaaranese hero. There are many legends about Morheka, however, each varying in the degree of believability and plausibility, so any truth to the legend must be regarded with suspicion.
Exactly what the tower contains inside it is also a matter of legendary speculation, and suggestions have ranged from weapons caches, to repositories of Forerunner knowledge, and even to hidden treasure. More recently, Voltakran researchers have suggested that it was part of a planetary communication and translocation grid, manipulating slipspace to send audio or visual messages and transport personnel and equipment as needed.
UNSC analysts sent by the Office of Naval Intelligence have had some luck, and the tower seems to respond to a human presence. It is not known yet whether this is due to their unique "Reclaimer" status, or whether the tower was "activated" by the events of late 2552.