Astroengineering

The Forerunners are renowned for their mimicry of natural environments, but it is their engineering achievements that stand as monuments to their extraordinary prowess, centuries after they have passed on. Their Halo Installations, Shield Worlds, and the Ark are all artificial habitats on a vastly larger scale than anything that either humanity or the Covenant could have accomplished. All known superstructures are a part of the Halo Array.

Orbitals
The Halo Installations are planet-sized artificial worlds, shaped like a ring. The interior surface contains a breathable atmosphere and seemingly natural terrain, including oceans, lakes, mountains, forests, and tundras. Underneath this is a support layer, upon which this terrain has been put in place, consisting of a vast grid network of metallic plates that provide the base. Though centrifugal forces may provide at least some of a Halo installations gravity, it also uses artificial gravity generators to generate Earth-like gravity.

The exact size of a Halo seems to vary. Alpha Halo has a diameter approximately equal to that of Earth, but Delta Halo was described as being only as large as a moon. Because these are the only two known Halo Installations, it remains to be seen whether these are standard sizes or whether each Installation is different in size and structure.

The Halo's were constructed as weapons of mass destruction on a galactic scale, designed to kill all sentient lifeforms in their radius of effect, approximately 25,000 light-years. The exact means by which they do this is unknown, but the fact that the Flood that was imprisoned upon them survived the previous activation may mean that those on its surface may be immune from the Halo's effect. Their habitability lends further credence to this theory.

Artificial Planets
The Forerunners were also adept at creating artificial planets, such as Onyx. Internally, the structure is maintained by, and even composed of, trillions of Sentinels and Production Factories, while externally the planet can appear like any other, with a wide variety of terrain.

The only known artificial planet, Onyx, was used as a cover for the Shield World in that system. It was later colonised by the UNSC, oblivious as to its true origins until 2552, when its Sentinels activated to defend the Shield World. It was later damaged by FENRIS Nuclear warheads, and destroyed as its Sentinels burst free and set up defensive stations around the Shield World portal.

Micro-Dyson Spheres
The Forerunners also constructed massive Dyson Spheres, with a 2 AU diameter, encapsulating them in a slipspace field to protect them from the Halo Array as it activated. Essentially a sphere, with its habitable surface on its internal surface rather than external, Shield Worlds are also equipped with their own stars, providing warmth, light and energy to the installation. Originally intended as a way to circumvent the Halo effect, the Shield Worlds were not put into use by the Forerunners due to the rampant actions of one of their own AI constructs.

Alderson Disks
The only known Alderson disk constructed by the Forerunners is Installation 00. With a diameter of between 90,000 and 100,000 kilometers, and a larger total surface area than a Halo Installation, it incorporates many similar elements, including Earth-like gravity and atmosphere and a rich and diverse ecosystem. Located in the center of the Installation is a small planetoid, presumably used for raw materials, and a star orbiting nearby despite its location: 218, or 2 trillion, lightyears outside of the Milky Way Galaxy, where stars cannot naturally form.

Rather than as a superweapon, like the Halo's, the Ark functions as both a control center for the entire array and as a construction facility for Halo's, using the planetoid to provide metals, gases, elements and other resources needed for reconstruction. After the construction is complete, the Halo is transported to the site or its predecessor via a slipspace portal.