Phoenix Logs/Forerunner/Forerunner Control Tower

From Halopedia, the Halo wiki

Multi-purpose control structures

As the Ark's name suggests, it was designed to play host to an incredible variety of life collected from across the galaxy. This almost inconceivable volume of biodiversity necessitated a flexible environmental system on the Ark as the Forerunners prepared for the galactic reseeding of life after the Halo Array was activated.

On a practical level this meant that all habitable surface area on the Ark could be covered in an environment to match any life-sustaining planet in the galaxy.

Atmospheres, soil and vegetation makeups, bodies of water or other liquids, and even gravitational conditions could be custom-set in areas as large or as small as needed to sustain collected specimens from a particular planet.

These environmental settings were accomplished through a network of Control Towers. These towers worked essentially like the output nodes of modern mass-particle replicators or the archaic 3D printers of the 22nd century, generating kilometers-deep environments around them through manipulation of raw materials stored deep within the superstructure of the Ark. The base of each tower then generated desired gravitational conditions.

When an environment was complete, only a small percentage of each Control Tower remained visible above ground in order to minimize interference with the new local wildlife while still allowing for data gathering on the surrounding area.

Today, these Control Towers lie mostly dormant. But with new visitors to the Ark and the possibility of new uses for the Ark's scientific and military capabilities, the towers offer literally world-building power to whoever controls this ancient installation.