Megastructure

Megastructures are feats of construction on massive scales. They can be grouped into six rough categories based on scale. 
== Surface-Class ==
Surface mega-construction exists entirely on the surface or within the atmosphere of planets. 
=== Old Earth Mega-Construction ===
Structures such as The Great Wall of China, a series of ancient fortifications spanning over 6300 kilometers, or 3900 miles, and the largest of ground-based particle accelerators are considered to be early feats of mega-construction by humans. 
Megalopoli, locations where cities have merged through expansion, are also considered early mega-construction. Asia alone featured thirty-six, which is unsurprising considering its size and population at the time. North and South America featured fifteen each for a total of thirty, Africa featured five, and Europe and Oceania each featured four for a total of seventy-nine megalopoli.
=== Megadam ===
Megadams would be used to create or drain seas for various purposes, whether to create new habitable land or decrease the temperature of planets. The most famous example of this structure is Atlantropa, a proposition that would terraform parts of Earth's surface by constructing massive hydroelectric dams in five locations, with the primary dam built across the Strait of Gibraltar.
This would create new land area for human habitation in both the Mediterranean Sea and northern Africa by lowering the sea level of the Mediterranean and refilling/expanding Lake Chad, expanding the African savannah northwards. In addition, this would produce massive amounts of efficient and clean electrical energy. 
=== Intercontinental Highway ===
Superhighway networks would be used to cross large bodies of water or terrain to connect regions to one another with massive roads spanning thousands of kilometers. Proto-superhighways include roads and bridges connecting mainland Europe to the Nordic countries across Denmark and China's national express highway network. 
=== Tensegrity Sphere ===
Tensegrity Spheres, also known as Cloud Nine or Balloon Habitats, are geodesic spheres built from high strength, low weight metals. They are filled with air and heated to be slightly warmer than the surrounding atmosphere, making them buoyant enough to suspend platforms or structures that may be used for various purposes. Given the nature of three-dimensional shapes, they gain volume at a much greater rate than they gain surface area as they become larger. This makes a carbon fiber and canvas sphere with a diameter of 1.6 kilometers lighter than the air contained within it, allowing it to float easily if heated. 
These were some of the earliest modern superstructures built by both humans and neyanta. The former used them to build early habitats suspended in Venus' atmosphere while the latter used them to expand the livable area on their colonies by planting them above the oceans. 
== Transorbital-Class ==
Transorbital mega-construction exists within the atmosphere and in low orbit of a planet.
=== Skyhook ===
Skyhooks are space stations with long tethers that extend downwards into a planet's atmosphere. They are capable of launching objects into orbit, and maintain their own via rocket or arcjet propulsion. These are widely used to skim hydrogen and helium from the upper atmosphere of gas giants. 
=== Space Elevator ===
Space Elevators are made from high-tensile strength cables extending from a planet's surface to a station outside of the geostationary orbit distance. This creates tension via centripetal force, and prevents the cable from buckling or collapsing back onto the planet. They allowed material to be transferred to space without the use of rockets. 
Humans built their first on the far side of Luna using Kevlar cables, later constructing a second on Mars from the same materials. 
=== Launch Loop ===
Launch Loops are structures in the range of 2000 kilometers long, suspended 80 kilometers above the ground by gyroscopes and tethered by cables and magnets to the ground. They would be used to launch vehicles into orbit via magnetic acceleration. 
== Orbital-Class ==
Orbital mega-construction orbits planets or stars directly, and are smaller than planets. 
=== Orbital Ring ===
Orbital rings are a marriage of Space Elevators and Launch Loops, with the loop suspended from space elevators instead of being tethered to the ground. 
=== Wheel Station ===
Wheel Stations are toroidal space stations that rotate to create artificial gravity. They have habitable surfaces on their interiors to support habitation. Many designs feature spokes and a central hub used to dock for supply transfer. The Unem have produced a number of wheel stations ranging in size from only a few hundred meters to many kilometers in diameter, all orbiting Demet and its moons. 
=== Sphere Station ===
Sphere Stations, or Bernal Spheres, are habitats with a number of sections. The center is composed of a spherical section that rotates to produce artificial gravity, with artificial terrain on its inner surface. Windows near the poles allow sunlight inside, where it is dispersed using mirrors. Past the poles are agriculture sections full of hydroponic farms. At the ends are sections with reactors, communication antennas and dishes, solar panels and radiators, and other equipment used to maintain the station. 
=== Cylinder Station ===
Cylinder Stations, or O'neill Cylinders, are composed of two counter-rotating cylinders with alternating bands of artificial terrain and thick glass windows in metal frames. Aligned with the glass windows are mirrored surfaces, allowing sunlight to enter the structure. The structures are to be angled towards the sun at all times, with the hinged end of the mirror facing away. To simulate nighttime, the mirrors are opened, allowing heat to be radiated away and not allowing sunlight to reach the interior of the structure. 
A similar structure was Liveship Traveler, the colony vessel used to carry humans from Earth to Revant in 2905. it was smaller than many proposed designs for such stations. 
=== Hollowed Asteroid ===
Hollowed Asteroids, also called Terraria, are habitats built inside of rotating asteroids filled with air. Some proposals involve melting the asteroid and inflating it for a more uniform shape. 
=== Orbital Ringworld ===
Orbital Ringworlds are the smallest scale ringworlds. Similar to wheel stations, they are ring-shaped habitats that rotate to produce artificial gravity. However, these are far larger, and are capable of maintaining their atmosphere without ceilings. These orbit planets, typically gas giants or large terrestrial planets. 
== Planetary-Class ==
Planetary mega-construction orbit stars directly, and are equal to or larger than planets in size.
=== Circumplanetary Ringworld ===
Circumplanetary Ringworlds are the middle scale ringworlds. They are large ring-shaped stations that maintain an atmosphere via its side walls and simulate gravity through rotation. Their diameters are equal to or greater than that of planets, and they orbit stars or encircle planets. Its rotation, combined with a significant tilt, simulates a day-night cycle. 
=== Global Cassus ===
A global cassus is built using a planet's entire mass to produce a much larger hollow structure with many times more habitable surface area. Equatorial areas are used for human habitation due to their comfortable gravity, while tropical regions would be dedicated to industrial concerns and agriculture, and polar regions would lack gravity entirely, instead being used for storage and other production where microgravity would assist. If Earth was used to build one, it would only be slightly smaller than Saturn. 
=== Shellworld ===
Shellworlds can refer to many structures composed of hollow shells. These are...
# A shell built around a barren body incapable of supporting its own atmosphere. The shell is supported by and contains the atmosphere. 
# A hollow planet supported by gas pressure from the inside. It may also have an atmosphere on the outside. 
# A nested series of hollow planets.
=== Planetary Supercomputer ===
Planetary supercomputers are perhaps the best known planetary-scale or larger megastructures in the galaxy, with two known to exist. Malish, an incomplete example orbiting Temno, and Empress, a complete but defunct example. These structures featured extremely deep geothermal energy harvesting equipment used to power supercomputers surrounding planets, covering the entire surface. The known examples also functioned to maintain the atmosphere as well.
=== Jupiter Brain ===
A Jupiter brain is a series of shells built around a gas giant, taking advantage of the radiation given off by the planet itself and waste heat emitted by one another to power massive supercomputers.
== Stellar-Class ==
Sellar mega-construction exist encircling or encapsulating stars, making them the largest feasible megastructures. 
=== Dyson Swarm ===
A Dyson swarm is composed of a large series of satellites and habitat stations designed to take advantage of a star's energy output. Wireless power transmission may be used to power planet-based facilities without obscuring enough of a star to significantly alter the surface brightness of planets orbiting it. 
=== Dyson Bubble ===
A Dyson Bubble is similar to a swarm, but the individual stations do not have normal orbital motion. They are instead statites, tethered to one another and held aloft with solar sails, the solar wind providing more energy than the star's gravity, canceling the descent of the objects in the bubble. 
=== Stellar Engine ===
Stellar Engines are structures designed to take full advantage of stellar radiation to produce various effects. 
==== Class A ====
Class A Stellar Engines are known as Shadkov thrusters, massive mirrors that deflect solar radiation in one direction. This causes the mirror to move away from the star. Due to the large mass of the mirror, the star is gravitationally drawn towards it as well. This creates a state of equilibrium that is capable of moving entire star systems over the course of millions of years. 
==== Class B ====
Class B Stellar Engines are known as Dyson spheres. Solid objects held in equilibrium between the star's gravity and solar wind pushing outwards. It captures all of a star's energy output for the builders. 
==== Class C ====
Class C Stellar Engines combine features of both Dyson spheres and Shadkov thrusters, capturing massive amounts of solar radiation while also being capable of moving star systems. 
=== Stellar Supercomputer ===
Stellar Supercomputers, also known as Matrioska brains, are composed of a large number of Class B Stellar Engines used to power massive supercomputers. Inner spheres take direct advantage of the star's radiation output, while outer spheres function based on the waste heat from spheres inward of it, creating a temperature gradient, from near the surface temperature of the star to near the temperature of interstellar space. This is similar to a Jupiter brain, but on a much larger scale. 
=== Circumstellar Ringworld ===
A circumstellar ringworld is the largest feasible ringworld, encircling a star and residing in the habitable zone for the constructing species. Construction requires the destruction of all planets in the system to prevent gravitational disturbances and provide material to be used in its construction.
=== Circumstellar Diskworld ===
Circumstellar Disks are by far the largest feasible megastructures, a massive disk spanning the entire habitable zone of a star. The star would be "orbiting" the disk by bobbing up and down in a hole at the disk's center. This would provide heat and light to the disk and allow for a day-night cycle to take place. 
=== Tropopilis ===
Tropopoli are similar to Cylinder Stations, but can be trillions of kilometers long, leading to the nickname "cosmic spaghetti". They can be looped around stars many times and house trillions of life forms. 
== Galactic-Class ==
=== Galactic Ringworld ===
A galactic ringworld would be the largest ringworlds ever conceived of that would be even slightly possible to construct on timescales of millions of years. It would be a ringworld habitat with a diameter on the scale of multiple light years, with multiple stars orbiting sections of the ring to provide light and heat.