Temple (Installation 05)
From Halopedia, the Halo wiki
The Temple structure that the Prophet of Regret occupied during his brief stay on Installation 05 was located in the center of a massive, Forerunner-constructed lake. The only way to the temple without a dropship or aircraft was by taking a route through a series of underwater tunnels, over bridges and by traveling on anti-gravity gondolas. The purpose of the temple-structure was never discovered because it was destroyed by a Covenant Battle Cruiser shortly after the Master Chief disposed of the Prophet of Regret. However, if the player takes time through the start of the level Regret, they are able to hear Cortana's various thoughts on the structures. Such comments made are, "This lake couldn't have been formed by volcanic action, so it was either built this way on purpose or formed through some other cataclysmic event," and, "I think the Forerunners built these new structures over the old ones, to protect them, honor them."
Some speculate that the ruins are transplanted Precursor buildings or transferred structures from the Forerunners original homeworld.
