A Megastructure is an extremely large built structure. Some define it as a building more than 1,000 km long or tall, though this is not a universal definition.
Only a handful of real world structures qualify:
If you stretch the definition (a lot) you could include things like the Transcontinental Railroad, the Nazca Lines, some Skyscrapers from the list of the largest buildings in the world, or entire Cities.
Proposed, Hypothetical, and Science Fictional Megastructures:
- Terrestrial Megastructures:
- Arcology — May not qualify due to not reaching the size minimum.
- Monument to the Third International — Significantly under the 1,000 km limit, but it's huge and impractical and looks like a plot device.
- Trans Global Highway — Technically doesn't qualify because it's made up of a network of smaller components constructed independently
- Transorbital Megastructures:
- Orbital Megastructures:
- Planetary Scale Megastructures: See also Stellar Megastructure
- Interstellar Scale Megastructures: See also Stellar Megastructure
- Alderson Disk
- Dyson Sphere
- Niven Ring
- Matrioshka Brain
- Rama (Spacecraft)
- Stellar Engine
- Shkadov Thruster
- Tipler Cylinder
Game and Story Use
- Many of these also qualify as Interstellar Terrain.
- A fun bit of background scenery and flavor in a Twenty Minutes Into The Future game could be the early stages of the construction of one of these on or near earth.
- The Precursors could have built one of these in deep space. Once found and boarded, there's evidence they were also the Ancient Astronauts who shaped the Evolution of man.
- Our Universe has more Dark Matter than normal matter, but science is divided on what the dark matter is. Perhaps the galaxy is populated by a vast civilization that lives in stealthy Dyson Spheres, and interact or travel via miniature wormholes. Normally, there is no stealth in space, but a 100% efficient Dyson Sphere might achieve just that. Of course, such a society would score very high on the Kardashev Scale.
- Implementing any of these structures, especially the Stellar Megastructures, will make Sufficiently Advanced Aliens that much more impressive. It's huge on a grand and epic scale.