A long time ago a large asteroid crashed into a dry rocky planet.

When an intelligent species explored its solar system, they discovered an energy source within the asteroid.
When they opened it, the species found a glowing crystal within it. In reaction with the asteroid's rock, water began to flow out of the rock.
They could´t find a scientific explanation for this phenomenon and decided to carve a monument out of the asteroid's rock.
From the remains they built a city around the monument.
When they were finished, the crater had turned into a green oasis.

Today, the species disappeared a long time ago, but its legacy will remain for eternity.