Twenty-five thousand years ago, an alien Kree patrol ship in the Solar System shot down a ship manned by Eternals, a human subspecies created on Earth by the Celestials. Kree scientists studied the remains of one Eternal, Arlok, and were fascinated by his superhuman powers. Realizing that Earth beings possessed unique genetic potential, the Kree sent a scientific party to Earth to perform experiments upon humans with the intention to create an army of superhumans for themselves. Their creations possessed enhanced strength and long lifespans. However, the Kree ultimately abandoned their work on Earth for unknown reasons, and the Inhumans developed into a distinct society.
The Inhumans, led by King Myran, settled in the city of Attilan, located in the North Atlantic Ocean just off the island continent of Atlantis, and became one of the most advanced cultures on Earth. When the Great Cataclysm destroyed Atlantis, Attilan endured and became legendary amongst humanity. They surrounded their city with a protective dome, subtly altering its internal environment, and began referring to Attilan as the Great Refuge. Not long after the Cataclysm, the geneticist Randac discovered the Terrigen mist, a gaseous mutagenic substance created by the exposure of rare Terrigen crystals to precise combinations of heat and vapor. These crystals appeared to only be found in a cavern beneath Attilan.