Martian (Invader Zim)

"Because it's cool!"

- A Martian hologram explaining why the people of Mars decided to turn their planet into a giant spaceship

The Martians (or Marzoids, as Zim calls them) are a long extinct civilization that once inhabited the planet of Mars.

History
They built pyramids, similar to those built by Earth's ancient Mesoamerican cultures. Before going extinct, the Martians managed to convert their whole planet into a huge spaceship, apparently just "because it's cool!". The mysterious structure known as the Face of Mars is where the ship's controls are located. Before turning their homeworld into a spaceship, the Martians tested the same technology on a small nearby planet which turned out to be Mercury.